#include <stdio.h>

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

long euclid(long d, long dd, long *p, long *q)
{
	long 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;
}

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

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