#include <iostream.h>

// sequence is 0, 1, 1, 2, 3, 5, 8, 13, ...

int fib (int i) {
  int pred, result, temp;
  
  pred = 1;
  result = 0;

  while (i > 0) {
    temp = pred + result;
    result = pred;
    pred = temp;
    i = i-1;
  }
  return(result);
}

int main () {
  int n;
  cout << "Enter a natural number: ";
  cin >> n;
  while (n < 0) {
    cout << "Please re-enter: ";
    cin >> n;
  }
  cout << "fib(" << n << ") = " << fib(n) << endl;
  return(0);
}