#include <stdio.h>

int main(void)
    {
    int num,a,b,c,r,k;
    while (1)
	{
	scanf("%d %d %d %d\n",&a,&b,&c,&k);
	if (!(a||b||c||k)) return 0;
	r = (b-a)>=0?-1:0;
	while (1)
	{
	r++;
	if (!((((b-a))+(r*(1<<k)))%c))
	    {
	    num = ((int)(((b-a))+(r*(1<<k)))/c);
	    if (num == 1 && r != 0)
	        printf("FOREVER\n");
	    else 
		printf("%d\n",num);
	    break;
	    }
	else
	    {
	    if (!((1<<k)%c))
		{
    	        printf("FOREVER\n");
	        break;
		}
	    }
	}
	}
    }