#include <stdio.h>

unsigned int A,B,C,k,x,y;
unsigned int base;
unsigned int sum;
unsigned int check;


int main() {

    while (1) {
    
	scanf("%d %d %d %d\n",&A,&B,&C,&k);
	if (A==0 && B==0 && C==0 && k==0) break;
	base = 1 << k;
	y = B - A; x = 0;
	if(y < 0) x = (base-A)/C;
	while (y<0) y = y + base;
	while (y>=base) y = y - base;
	
	check = 0;
	sum = x*C;
	for(;x<base;x++) {
	  if (sum == y) { check = 1; printf("%d\n",x); break; }
	  sum += C;
	}
	if (check==0) printf("FOREVER\n");
    
    }

    return 0;
}