#include <stdio.h>
#include <map>
using namespace std;

long long vys,a,b,c,k,zv,x,p,posun,sp;
map <long long, char> m;

void ries2() {
	int times;
	posun=1ll<<k;
	b=(b-a+posun)%posun;
	p=0;
	zv=c-(posun%c);
	sp=posun/c+1;
	if (b%c==0) printf("%lld\n",b/c);
	else {
		m.clear();
		x=0; times=0;
		while (m[x]!=(char)1 && x!=b%c) { m[x]=(char)1; x=(x+zv)%c; ++times; }
		if (x!=b%c) printf("FOREVER\n");
		else {
			p=times*sp;
			p+=(b-x)/c;
			printf("%lld\n",p);
		}
	}
}

void ries3() {
	long long times=0;
	m.clear();
	posun=1ll<<k;
	b=(b-a+posun)%posun;
	zv=c-(posun%c);
	sp=posun/c+1;
	x=0; p=0;
	while ((b-x)%c && !m[x]) {
		m[x]=(char)1;
		x=(x+zv);
		times=(times+sp)-((x-1)/c);
		x%=c;
	}
	if ((b-x)%c) printf("FOREVER\n");
	else printf("%lld\n",times+(b-x)/c);
}

int main() {
	while (scanf("%lld %lld %lld %lld",&a,&b,&c,&k)!=EOF) {
		if (!a && !b && !c && !k) return 0;
		if (c==0) {
			if (a==b) printf("0\n");
			else printf("FOREVER\n");
			continue;
		}
		ries3();
/*		m.clear();
		while (x<posun && abs(b-x)%c) {
			if (m[x]) break;
			m[x]=(char)1;
			x=(x+zv);
			p+=sp;
		}
		if (abs(b-x)%c==0) {
			p+=(b-x)/c;
			printf("%lld\n",p);
		} else printf("FOREVER\n");*/
	}
	return 0;
}