diff --git a/nessesary/parser/parser.py b/nessesary/parser/parser.py
new file mode 100644
index 0000000..fe8223b
--- /dev/null
+++ b/nessesary/parser/parser.py
@@ -0,0 +1,173 @@
+def add_neg_sign(expr: str, is_neg: bool) -> str:
+    """
+    Add negative sign to expression if 
+    is_neg is True.
+    """
+    if is_neg:
+        expr = "-" + expr
+    return expr
+
+def is_neg(expr, i) -> bool:
+    """
+    Check if input expression is negative or not.
+    """
+    n = len(expr)
+    neg_sign = 0
+    while i < n and expr[i] == "-":
+        neg_sign += 1
+        i += 1
+    i -= 1
+    return neg_sign%2 != 0
+
+def is_num(char: str):
+    """
+    Check if char agrument is number or not.
+    """
+    return char in ['1','2','3','4','5','6','7','8','9','0']
+
+def is_op(char: str):
+    """
+    Check if char agrument is operator or not.
+    """
+    if char in ["+","-","*","/","^","("]:
+        return True
+    return False
+
+def is_letter(char: str):
+    """
+    Check if char agrument is string character or not.
+    """
+    return "a" <= char <= "z"
+
+def is_func(expr: str, i: int, result: str):
+    """
+    Check if expression is a function or not.
+    """
+    result = ""
+    for i in range(len(expr)):
+        if not is_letter(expr[i]):
+            i -= 1
+            break
+        result += expr[i]
+    return len(result) > 1
+
+def priority(char: str):
+    """
+    set precedence for each operator.
+    This program use PEMDAS RULE as main rule 
+    to implement.
+    """
+    if char == "+" or char == "-":
+        return 2
+    elif char == "*" or char == "/":
+        return 3
+    elif char == "^":
+        return 4
+    return 5
+
+def parsing_num(expr: str, i: int):
+    """
+    Find the first number after the index i.
+    Return string of number and the end index of it.
+
+    >>> parsing_num("123+456", 0)
+    ("123",2)
+    """
+    n = len(expr)
+    start = i
+
+    while (i < n) and (is_num(expr[i]) or expr[i] == "."):
+        i += 1
+    return expr[start:i], i - 1
+
+def make_postfix(expr: str):
+    """
+    Change infix notation intoPostfix Notation
+    Postfix Notation has a better ability to 
+    set priority of each operand.
+    
+    >>> make_postfix(" 3*4 + 2*5")
+    3 4 * 2 5 * +
+
+    >>> make_postfix("(2-3+4)*(5+6*7)
+    2 3 - 4 5 6 7 * + ( * + (
+    """
+
+
+    if len(expr) == 0:
+        return ""
+
+    op_stack = []
+    result = ""
+    temp = ""
+    neg = False
+    i = 0
+    while i < len(expr):
+        char = expr[i]
+        if char == " ":
+            i += 1
+            continue
+
+        if is_num(char):
+            num, i = parsing_num(expr, i)
+            result += add_neg_sign(num, neg)
+            neg = False
+            result += " "
+            i += 1
+            
+
+        elif char == ")":
+            found = False
+            while len(op_stack) != 0:
+                head = op_stack.pop()
+                if head == "(":
+                    found = True
+                    break
+                else:
+                    result += head
+                    result += " "
+            i += 1
+            if not found:
+                raise ValueError("Parentheses Error!")
+
+        elif is_op(char) or is_func(expr, i, temp):
+            isfunc = len(temp) > 1
+            if (not isfunc) and (char == "-"):
+                if i == 0 or is_op(expr[i-1]):
+                    neg = is_neg(expr, i)
+                    continue
+
+            while len(op_stack) != 0:
+                head = op_stack[-1]
+                priority1 = (priority(head) < priority(char))
+                priority2 = (priority(head) == priority(char))
+                if char == "(" or head == "(" or priority1 or (priority2 and char == "^"):
+                     break
+                result += head
+                op_stack.pop()
+                result += " "
+
+            if isfunc:
+                op_stack.append(add_neg_sign(temp, neg))
+                neg = False
+                temp = ""
+            else:
+                op_stack.append(char)
+            i += 1
+
+        elif is_letter(char):
+
+            result += add_neg_sign(char, neg)
+            neg = False
+            result += " "
+            i += 1
+
+        else:
+            raise ValueError("Can't Parse this Letter!")
+        
+    while len(op_stack) != 0:
+        head = op_stack.pop()
+        result += head
+        result += " "
+    i += 1
+    return result[:len(result)-1]
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