不过还是没1A，注意有a==b,c=0的情况

#include <cstdio>
typedef long long ll;

ll a, b, c, k, d, m;

ll extEuc(ll m, ll k) {
	long long *u, *v, *t, a[3][3] = {1, 0, m, 0, 1, k}, q;
	u = (m > k) ? a[0] : a[1];
	v = (m > k) ? a[1] : a[0];
	t = a[2];
	while(v[2] != 1) {
		q = u[2] / v[2];
		t[0] = v[0], t[1] = v[1], t[2] = v[2];
		v[0] = u[0]-q*v[0], v[1] = u[1]-q*v[1], v[2] = u[2]-q*v[2];
		u[0] = t[0], u[1] = t[1], u[2] = t[2];
	}
	while(v[1] < 0) v[1] += m;
	return v[1] % m;
}

int main() {
	while(scanf("%lld%lld%lld%lld", &a, &b, &c, &k) && k) {
		m = 1LL << k, d = (b - a + m) % m;
		if(!d && !c){
			printf("0\n");
			continue;
		}
		while(!(d&1) && !(c&1))
			d >>= 1, c >>= 1, m >>=1;
		if(!(c&1)) {
			printf("FOREVER\n");
			continue;
		}
		printf("%lld\n", extEuc(m, c) * d % m);
	}
	return 0;
}