#include <iostream.h>
#include <fstream.h>
long long  A,B,i,K,C,nrc=0,cf,nr=0,v[100000000];
int main()
{
    ifstream f("cifre.in");
    ofstream g("cifre.out");
    f>>A>>B>>K>>C;
    f.close();
    for (i=A;i<=B;i++)
        v[i]=i;
    for (i=A;i<=B;i++)
        {
        nr=0;
        do
          {
          cf=v[i]%10;
          v[i]/=10;
          if (cf==C) nr++;
          }
        while(v[i]);
        if (nr>=K) nrc++;
        }
    g<<(float)nrc/B;   
    g.close();
    return 0;
}