#include<stdio.h>
long a,b,c,k;
int cif(int x)
{ int aux=0;
  while(x)
    { if(x%10==c) aux++;
      x=x/10;
      if(aux==k) return 1;
    }
  return 0;
}
int main()
{ freopen("cifre.in","r",stdin);
  freopen("cifre.out","w",stdout);
  scanf("%d %d %d %d",&a,&b,&c,&k);
  int aux=0;
  for(long i=a;i<=b;i++)
      if(cif(i)) aux++;
  printf("%.4f",(float)aux/(b-a+1));
  fclose(stdin);
  fclose(stdout);
  return 0;
}