#include <iostream>
#include <cstdio>
typedef long long LL;
using namespace std;

LL exp(LL a, LL b)
{
    LL res = 1;
    while (b)
    {
        if (b & 1)
            res *= a;
        a *= a;
        b >>= 1; 
    }
    return res;
}

LL exgcd(LL a, LL b, LL &x, LL &y)
{
    if (!b)
    {
        x = 1, y = 0;
        return a;
    }
    LL res = exgcd(b, a%b, y, x);
    y -= a/b * x;
    return res;
}

int main()
{
    LL a, b, c, k, x, y;
    while (scanf("%lld%lld%lld%lld", &a, &b, &c, &k) && a+b+c+k)
    {
        LL p = exp(2, k);
        LL gcd = exgcd(c, p, x, y);
//      printf("%lld*%lld + %lld*%lld = %lld\n", c, x, p, y, gcd);
        if ((b-a)%gcd)
            puts("FOREVER");
        else
        {
            x *= (b-a) / gcd;
            p /= gcd;
            printf("%lld\n", (x%p+p)%p);
        }
    }
    return 0;
}