#include <stdio.h>
typedef long long ll;

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

ll euclid(ll d, ll dd, ll *p, ll *q)
{
	ll x, y, xx, yy;
	x = 1;
	xx = 0;
	y = 0;
	yy = 1;
	do {
		if (d > dd) {
			x -= xx * (d/dd);
			y -= yy * (d/dd);
			d %= dd;				
		} else {
			xx -= x * (dd/d);
			yy -= y * (dd/d);
			dd %= d;				
		}
	} while (d != 0 && dd != 0);

	if (d == 0) {
		if (p) *p = x;
		if (q) *q = y;
		d = dd;
	} else {
		if (p) *p = xx;
		if (q) *q = yy;
	}
	/*
	if (d == 0) {
		if (p) *p = xx;
		if (q) *q = yy;
		d = dd;
	} else {
		if (p) *p = x;
		if (q) *q = y;
	}
	*/
	return d;
}

ll inverse(ll x, ll m)
{
	euclid(x,m,&x,NULL);
	if (x < 0) x += m;
	return x;
}

ll div(ll a, ll b, ll m)
{
	ll x;
	ll inv;

	if (b == 1) return a % m;
	inv = inverse(b,m);
	dp(("inv %lld %lld = %lld\n",b,m,inv));
	
	x = (a*inv)%m;
	if (!x) x = a/b;
	return x;
}
int main(void)
{
	ll a, b, c, k,m ,u,v;
	
	while (1) {
		scanf("%lld%lld%lld%lld",&a,&b,&c,&k);
		if (a == 0 && b == 0 && c == 0 && k == 0) break;
		if (a == b) {
			puts("0");
			continue;
		}
		if (!c) { puts("FOREVER"); continue; }
		m = 1ll << k;
		u = (b - a + m) % m;
		v = div(u,c,m);
		dpf(("uv: %lld %lld %lld\n", u , v,m ));
		if ((v * c) % m != u)
			puts("FOREVER");
		else 
			printf("%lld\n",v);
	}
	return 0;
}