#include <iostream>
#include <iomanip>
using namespace std;
int a,b,c,k,k1,l,i,i1;
double p;
int main()
{
    cin>>a>>b>>c>>k;
    for(i=a;i<=b;i++,k1=0){
            i1=i;
    while((i1>0)&&(k1<k)){if(i1%10==c)k1++;i1/=10;}
    if(k1==k)l++;}
    p=l/((b-a+1)*1.00);
    cout<<setprecision(4)<<p;
    cout.flush();

    return 0;
}