Wrong

int s1, s2 ;

int fibonacci (int n)
{      if (n == 0) return 1;
  else if (n == 1) return 1;
  else {
         s1 = fibonacci(n-1);
         s2 = fibonacci(n-2);
         return s1 + s2;
       }
}

Right

int fibonacci (int n)
{ int s1,s2;
       if (n == 0) return 1;
  else if (n == 1) return 1;
  else {
         s1 = fibonacci(n-1);
         s2 = fibonacci(n-2);
         return s1 + s2;
       }
}