#include <stdio.h>

stru

int main() {
   unsigned k,i;
   unsigned long a,b,c,max;
   long variable;
   scanf("%li %li %li %d", &a, &b, &c, &k);
   while((a!=0) && (b!=0) && (c!=0) && (k!=0)) {
      max=1;
      for (i=1;i<=k;i++) max=max*2;
      variable=b-a;
      if (variable < 0) variable += max;
      if(variable % c!=0) printf("FOREVER\n");
      else printf("%li\n", variable/c);
      scanf("%li %li %li %d", &a, &b, &c, &k);
   }
   return(0);
}