C :

#include <stdio.h>
#include <math.h>

int main()
{
	long a, b, c, d;
	int k;
	while (scanf("%ld %ld %ld %ld", &a, &b, &c, &k), a || b || c || k)
	{
		d = b - a;
		if (d < 0)
		{
			d += powf(2, k);
		}
		if (d %c)
		{
			printf("FOREVER\n");
		}
		else
		{
			printf("%d\n", d/c);
		}
	}
	return 0;
}

C++ :

#include <iostream>
using namespace std;
typedef long long int64;

int64 mod(int64 a, int64 b){
    if (a < 0) return a % b + b;
    return a % b;
}

int64 ex_gcd(int64 a, int64 b, int64 &x, int64 &y){
    if (b == 0){
        x = 1; y = 0; return a;
    }else{
        int64 d = ex_gcd(b, a % b, x, y);
        int64 temp = x;
        x = y; y = (temp - a / b * y);
        return d;
    }
}

int main(){
    int64 a, b, c, k, A, B, C, x, y;
    while(cin>>a>>b>>c>>k, a || b || c || k){
        A = c; B = ((int64)1) << k; C = (b - a + B) % B;
        if (c == 0){
            if (C == 0) cout<<"0"<<endl;
            else cout<<"FOREVER"<<endl;
            continue;
        }
        int64 d = ex_gcd(A, B, x, y);
        if (C % d) cout<<"FOREVER"<<endl;
        else cout<<mod(C * x / d, B / d)<<endl;
    }
    return 0;
}