#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <ctype.h>

typedef long long ll;

/* TODO */
#if 0
#define dpf(x) printf x
#else
#define dpf(x) (void)0
#endif
#define dp dpf

#define adelib(a,b) (!((b) % (a)))

ll compute( ll diff, ll c, ll m )
{
	dpf(("x\n"));
	if (adelib(c,diff))
		return diff/c;
	if (adelib(c,m))
		dpf(("BOOM\n"));
	return ((m/c)+1) * compute( diff, c - (m % c), m );
}

int main(void)
{
	ll a, b, c, k, m , diff;
	while (1) {
		scanf("%lld %lld %lld %lld\n",&a,&b,&c,&k);
		if (a == 0 && b == 0 && c == 0 && k == 0)
			break;
		if (a == b) {
			puts("0");
			continue;
		}
		m = 1LL << k;
		diff = (b - a + m) % m;
		if (!c || (!adelib(c,diff) && adelib(c,m))) {
			puts("FOREVER");
			continue;
		}
		printf("%lld\n",compute(diff,c,m));
	}
	return 0;
}