#include<fstream.h>
int cifra(int x,int c)
{int k=0;
 while(x)
 {if(x%10==c)k++;
  x/=10;
 }
 return k;
}
int main()
{int c,k,i,nr=0,aux;
 long a,b;
 double p;
 ifstream f("cifre.in");
 ofstream g("cifre.out");
 f>>a;
 f>>b;
 f>>c;
 f>>k;
 if(a>b){aux=a;
         a=b;
         b=aux;
        }
 for(i=a;i<=b;i++)
  if(cifra(i,c)>=k)nr++;
 p=(double)nr/(b-a+1);
 g<<p;
return 0;
}