#include <stdio.h>

int main(void) {
  unsigned int a,b,c;
  int k;
  unsigned int x,pom,vel;
//  unsigned long vel;
  scanf("%d %d %d %d",&a,&b,&c,&k);
  while (!((a==0)&&(b==0)&&(c==0)&&(k==0))&&(b!=0)) {
    x=0; vel = 1;
    for (int i=0; i<k; i++) vel *= 2;
//    printf("%d",vel);
    pom=0;
    while ((a!=b)&&(pom!=2)) {
      if (a<b) pom=0;
      else pom=1;
      if (a+c<vel) a+=c;
      else a=a+c-vel;
      if ((pom==0)&&(a>b)) pom=2;
      x++;
    }
//    printf("HU");
    if ((pom!=2)&&(b!=0)) printf("%d\n",x);
    else printf("FOREVER\n");
    scanf("%d %d %d %d",&a,&b,&c,&k);
  }

  return 0;
}