#include <cstring>
#include <cstdio>
#include <cmath>
using namespace std;
typedef long long ll;
ll a,b,c,k;
ll _gcd(ll a,ll b,ll &x,ll &y)
{
    if(b==0)
    {
        x=1;
        y=0;
        return a;
    }
    ll ans=_gcd(b,a%b,x,y);
    ll tmp=x;
    x=y;
    y=tmp-(a/b)*y;
    return ans;
}
ll extend_gcd(ll a,ll b,ll c)
{
    ll x,y;
    ll gcd=_gcd(a,b,x,y);
    if(c%gcd)
        return -1;
    x=x*c/gcd;
    b/=gcd;
    ll ans=(x%b+b)%b;
    return ans;
}
int main()
{
    while(scanf("%lld%lld%lld%lld",&a,&b,&c,&k),a||b||c||k)
    {
        ll mod=(ll)1<<k;//WA!!!没有类型转换
        ll ans=extend_gcd(c,mod,b-a);
        if(ans==-1)
            printf("FOREVER\n");
        else
            printf("%lld\n",ans);
    }
    return 0;
}