String insertion at specified location

Aim:
To insert a substring into a string.

Program:
#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
   char str[80],substr[20];
   char X,newstr[80];
   int pos,count1=0,count2=0,
   int i=0,t=0,l;
   clrscr ();
printf("Enter a string:");
gets(str);
printf("Enter a substring:");
gets(substr);
while( str[i] != '\0' )
    {
       count1++;
       i++;
    }
while( substr[i] != '\0' )
    {
       count2++;
       i++;
    }
printf("\nEnter the position to insert the substring");
scanf("%d",&pos);
strcpy(newstr,str);
l = pos+count2;
for( i=pos;i<=count1+count2;i++)
  {
    X = newstr[i];
    if(t < count2)
     {
       str[i] = substr[t];
          t = t+1;
      }
      str[l] = X;
      l++;
   }
printf("\nNewstring is: %s",str);
}

Program:
Enter a string: hydai
Enter a substring: un
Enter the position to insert the substring: 3
Newstring is hyundai