//poj2115

#include<cstdio>

#include<cstdlib>

#include<cstring>

#include<iostream>

using namespace std;

typedef long long LL;

LL bit[];

LL tx,ty;

LL exgcd(LL a,LL b)

{

    if(b==) {tx=,ty=;return a;}

    LL d=exgcd(b,a%b);

    LL x=ty,y=tx-(a/b)*ty;

    tx=x;ty=y;

    return d;

}

int main()

{

    //freopen("a.in","r",stdin);

    //freopen("a.out","w",stdout);

    bit[]=;

    for(LL i=;i<=;i++) bit[i]=bit[i-]*;

    LL a,b,c,k;

    while()

    {

        scanf("%I64d%I64d%I64d%I64d",&a,&b,&c,&k);

        if(!a && !b && !c && !k) return ;

        LL A=c,B=bit[k],C=b-a;

        LL g=exgcd(A,B);

        if(C%g) printf("FOREVER\n");

        else

        {

            LL x=tx*(C/g);

            x=(x%(B/g)+(B/g))%(B/g);

            printf("%I64d\n",x);

        }

    }

    return ;

}