#include <iostream>
#include <math.h>

int main()
{
  int A, B, C, k;
  bool ano;
  unsigned long i;
  unsigned long pocet;
  
  
  do {
    cin >> A >> B >> C >> k;
    if((A==0) && (B==0) && (C==0) && (k==0)) {
      return 0;
    } //if
    
    if(A==B) {
      cout << 0 << endl;
      continue;
    } //if
    else if((B>A) && ((B-A)%C==0)) {
      cout << ((B-A)/C) << endl;
      continue;
    } //if
    
    ano=false;
    pocet = 0;
    for(i=A;i<((unsigned long)pow(2,k)*C);i+=C) {
      pocet++;
          if ((i%((unsigned long)pow(2,k)))==B) {
        ano=true;
	break;
      } //if
    } //for
    if(ano) {
      cout << pocet-1 << endl;
    } //if
    else {
      cout << "FOREVER" << endl;
    } //else
    /*
    else if((A>B) && ((B-A)%C==0)) {
      cout << ((int)pow(2,k)/C-((A-B)/C)) << endl;
      continue;    
    } //if
    else if((abs(A-B)%C)!=0) cout << "FOREVER" << endl;
    
    */    
  } while(1);

 
   return 0;
 }