#define _CRT_SECURE_NO_WARNINGS
#define _SILENCE_CXX17_C_HEADER_DEPRECATION_WARNING
#include <cassert>
#include <cstdio>
using namespace std;
typedef long long ll;
void exgcd(ll a, ll b, ll &d, ll &x, ll &y)
{
    b == 0 ? (x = 1, y = 0, d = a) : (exgcd(b, a % b, d, y, x), y -= x * (a / b));
}
int main()
{
    ll a, b, c, k;
    while (scanf("%lld%lld%lld%lld", &a, &b, &c, &k), a | b | c | k)
    {
        ll mask = (1ll << k) - 1;
        ll dif = (b + (1ll << k) - a) & mask;
        ll _g, x, y;
        exgcd(c, 1ll << k, _g, x, y);
        x = x * (dif / _g);
        x = (x % ((1ll << k) / _g) + ((1ll << k) / _g)) % ((1ll << k) / _g);
        if (dif % _g)
            puts("FOREVER");
        else
            printf("%lld\n", x);
    }
    return 0;
}