include <stdio.h>

char*  strstr ( char * str1, char * str2)
{
  char *cp = str1;
  char *s1, *s2;
  if ( !*str2 )
    return(str1);
  while (*cp) {
    s1 = cp;
    s2 = str2;
    while ( *s1 && *s2 && !(*s1-*s2) )
      s1++, s2++;
    if (!*s2)
      return(cp);
    cp++;
  }
  return(NULL);
}

int main()
{
  char s1[]="I am a boy";
  char s2[]="am";
  printf("%s", strstr(s1,s2));
  return 0;
}