#include <math.h>
#include <stdio.h>

int main(void)
{
int count,a,b,c,k,x;
while (scanf("%d %d %d %d",&a,&b,&c,&k)) {
count=0;
if (a==0 && b==0 && c==0 && k==0) return 0;
if (c==0) {
if (a==b) printf("0\n");
else printf("FOREVER\n");
return 0;
}
if ((a%c) != (b%c)) {
	printf("FOREVER\n");
	continue;
	}
if (a>b) {
    for (x=a;x!=b && (int) pow(2,k-1) <= count;x-=c) count++;
    printf("%d\n",(int)pow(2,k-1)-count);
    }
else {
    for (x=a;x!=b && (int) pow(2,k-1) <=count;x+=c) count++;
    printf("%d\n",count);
    }
}
return 0;
}