#include <library.h>

int fib(int N,  // I want the Nth fibonacci number
        int K,  // By the way, I already know what the Kth one is
        int P,  // The Kth fibonacci number is P
        int PP) // and the K-1th fibonacci number is PP
{ if (K == N)
    return P;
  return fib(N, W+1, P+PP, P); }

          // This is an unbelievably small function, but very
          // mysterious to the uninitiated. Those comments
          // are important. It is also very fast.

void main()
{ while (true)
  { cout << "Number (or negative to stop)? ";
    const int x = read_int();
    if (x < 0)
      break;
    if (x == 0)
    { cout << "0 is no good\n";
      continue; }
    const int y = fib(x, 2, 1, 1);
    cout << "fib(" << x << ") = " << y << "\n"; }
  cout << "Good-bye then\n"; }