#include<stdio.h>
#define LL unsigned long long
void exgcd(LL a,LL b,LL& d,LL& x,LL& y)
{
    if(!b){d=a;x=1;y=0;}
    else
    {
        exgcd(b,a%b,d,y,x);
        y-=x*(a/b);
    }
}
int main()
{
    LL A,B,C,k;
    while(scanf("%llu%llu%llu%llu",&A,&B,&C,&k),(A+B+C+k))
    {
        LL a,b,c,d,x,y,dm;
        c=B-A;
        if(c==0){printf("0\n");continue;}
        a=C;
        b=(LL)1<<k;
        exgcd(a,b,d,x,y);
        if(c%d){ printf("FOREVER\n");continue;}
        dm=b/d;
        x=(((x*c/d)%dm)+dm)%dm;

        printf("%llu\n",x);
    }
    return 0;
}