#include <stdio.h>

unsigned long long int inverse(unsigned long long int a, unsigned long long int b)
{
  unsigned long long int x;
  unsigned long long int res;
  unsigned long long int bit;
  
  x=a;
  res=1;
  
  for(bit=1; bit<b; bit++)
  {
    if(x&(1<<bit))
    {
      x+=a<<bit;
      res+=1<<bit;
    }
  }
  
  return res;
}


int main()
{
  unsigned long long int a, b, c, k, aa, bb, cc, mask;

  while(1)
  {
    scanf("%llu %llu %llu %llu ", &a, &b, &c, &k);
    if(!a && !b && !c && !k)
      break;
    
    mask=(1<<k)-1;
    
    a=(b-a)&mask;
    b=c;
    
    while(!(a&1) && !(b&1) && k)
    {
      a>>=1;
      b>>=1;
      k--;
    }
    
    if(!a)
      printf("0\n");
     else
    if(!(b&1))
      printf("FOREVER\n");
     else
    {
      mask=(1<<k)-1;
      
      printf("%llu\n", (inverse(b, k)*a)&mask);
    }
  }
  
  return 0;
}