#include<fstream.h>
int main()
 {
  long int a,b,c,k,i,v,d,n;
  double p;
  ifstream fin("cifre.in");
  ofstream fout("cifre.out");
  fin>>a;
  fin>>b;
  fin>>c;
  fin>>k;
  i=a;
  while(i<=b)
   {
    i=v;
    n=0;
    while(v!=0)
     {
      d=v%10;
      if(d=k)n=n+1;
      v=v/10;
      }
    i=i+1;
    }
  p=n/b;
  fout<<p<<'\n';
  fin.close();
  fout.close();
  return 0;
  }