/* fib4.c -- Fibonacci-Zahlen, interaktives Programm */

#include <stdio.h>

unsigned int fibonacci (unsigned int n)
{
  if (n == 0) return 0;
  else if (n == 1) return 1;
  else {
    unsigned int a = 0;
    unsigned int b = 1;
    unsigned int i = 2;
    while (i <= n) {
      /* Invariante: a == F(i-2) und b == F(i-1) */
      unsigned int sum = a + b;
      a = b;
      b = sum;
      i = i + 1;
      /* Wieder korrekt */
    }
    /* Hier ist i == n + 1, also b == F(n) */
    return b;
  }
}

int main ()
{
  unsigned int n;

  printf("Index n: ");
  scanf("%u", &n);
  printf("F(%u) = %u\n", n, fibonacci(n));
  
  return 0;
}