diff --git a/nessesary/parser/parser.py b/nessesary/parser/parser.py
index da79169..a710204 100644
--- a/nessesary/parser/parser.py
+++ b/nessesary/parser/parser.py
@@ -39,7 +39,7 @@ def is_letter(char: str) -> bool:
     """
     return "a" <= char <= "z"
 
-def is_func(expr: str, i: int, result: str) -> bool:
+def is_func(expr: str, i: int, result: str) -> tuple[bool, str, int]:
     """
     Check if expression is a function or not.
     """
@@ -103,6 +103,7 @@ def make_postfix(expr: str):
     result = ""
     temp = ""
     neg = False
+    has_func = False
     i = 0
     while i < len(expr):
         char = expr[i]
@@ -156,6 +157,7 @@ def make_postfix(expr: str):
                 neg = False
                 temp = ""
                 i = last_index
+                has_func = True
             else:
                 op_stack.append(char)
             i += 1
@@ -175,4 +177,88 @@ def make_postfix(expr: str):
         result += top
         result += " "
     i += 1
-    return result[:len(result)-1]
+    return result[:len(result)-1], has_func
+
+def insert_mul_sign(expr):
+    """
+    Insert multiplication sign to expression(*)
+    >>> insert_mul_sign("xx")
+    x*x
+    >>> insert_mul_sign("xy+xsin(xy)+12x+y")
+    x*y+x*sin(x*y)+12*x+y
+    """
+    function_list = [
+                    "sin","cos","tan", "cosec", "sec", "cot",
+                    "arcsin","arccos","arctan", "arcsec", 
+                    "arccosec", "arccot", "asin", "acos", 
+                    "atan", "asec", "acosec", "acot"
+                    ]
+    i = 0
+    result = ""
+    f_name = ""
+    neg = False
+    new = ""
+    for ind in range(len(expr)):
+        if expr[ind] == " ":
+            continue
+        new += expr[ind]
+    expr = new
+
+    while i < len(expr):
+        char = expr[i]
+        check_func, f_name, last_index = is_func(expr, i, f_name)
+
+        if is_num(char):
+            num, i = parsing_num(expr, i)
+            result += add_neg_sign(num, neg)
+            neg = False
+            i += 1
+            if expr[-1] != char:
+                if expr[i] != ")" and not is_op(expr[i]):
+                    result += "*"
+        elif char == ")":
+            try:
+                c = expr[i+1]
+                result += char
+                if c != ")":
+                    result += "*"
+            except IndexError:
+                result += char
+            i += 1
+
+        elif is_op(char):
+            result += char
+            i += 1
+
+        elif check_func:
+            check_list = [1 for n in function_list if n in f_name]
+            if sum(check_list) > 0:
+                result += f_name
+                i = last_index
+            else:
+                count = len(f_name)
+                for ind in range(len(f_name)+1):
+                    if ind % 2 != 0:
+                        f_name = f_name[:ind] + "*" + f_name[ind:]
+                result += f_name
+                i = last_index + count
+
+        elif is_letter(char) or char == "(":
+            if result[-1] != "*":
+                if (result[i] != "(") and (char != "("):
+                    result += "*"
+            result += char
+            # if not is_op(expr[i]):
+            #     result += "*"
+            i += 1
+
+    return result
+
+
+# print(make_postfix("(2-3+4)*(5+6*7)"))
+# print(make_postfix("12sin(x^2)+(4x+12*3)-5"))
+# print(insert_mul_sign("12x"))
+# print(insert_mul_sign("5x+12y+421abcde+1"))
+# print(insert_mul_sign("12((x+12x)x)"))
+# print(insert_mul_sign("xsin(x^2)"))
+# print(insert_mul_sign("5(1+2*3^12)(12-5(4^2)) + (1-3*4)12"))
\ No newline at end of file