#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() {
	posun=1ll<<k;
	b=(b-a+posun)%posun;
	p=0;
	zv=c-(posun%c);
	sp=posun/c+1;
	if ((b%c)%zv==0) {
		x=b%c;
		p=(b%c)/zv*sp;
		p+=(b-x)/c;
		printf("%lld\n",p);
	} else printf("FOREVER\n");
}

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;
		}
		ries2();
/*		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;
}