#include <stdlib.h>
#include <stdio.h>

int main(void){
  unsigned int A,B,C,i,j,k,t,u;
  long long k2;
  while(1){
    scanf("%d %d %d %d\n",&A,&B,&C,&k);
    if(!A && !B && !C && !k)break;
    i=0;j=C;
    k2=(long long)1<<((long long)k);
    while(1){
      if(A<=B && ( (B-A)%C==0) ){
	i+=(B-A)/C;
	 printf("%d\n",i);
	 break;
      }
      if(!j){
	printf("FOREVER\n");
	break;
      }
      /*      printf("%lld %d %lld %d %lld %d\n",k2,A,k2-A,C,(k2-A)/C,k);
       */
      t=(k2-A)/C;
      i+=t;
      u=(k2-A)%C;
      if(u)i++;
      A=C-u;
      /*printf("na==%d, nb==%d, nc==%d, t==%d, i==%d, u==%d\n",A,B,C,t,i,u);
       */
      j--;
    }
  }
  return 0;
}