/*
 * Program to compute elements of the Fibonacci sequence using loop.
 *
 * (more efficient than recursion, but also a little harder to write and 
 * understand?)
 */
#include <stdio.h>

long fib(int a);

int main(void) {

	int n;
	printf("enter n\n");
	if (scanf("%d", &n) != 1) {
		printf("not number\n");
		return 1;
	}
	printf("fib(%d) is %ld\n", n, fib(n));

	return 0;
}

long fib(int a) {
	if (a <= 1) {
		return 1;
	}
	long prev1 = 1; /* fib(a-1) */
	long prev2 = 1; /* fib(a-2) */
	long next;
	/* compute fib(n) for n = 2 ... a */
	for (int n = 2; n <= a; ++n) {
		next = prev1 + prev2;
		/* copy fib(a-1) to fib(a-2) */
		prev2 = prev1;
		/* copy fib(a) to fib(a-1) */
		prev1 = next;
	}
	return next;
}