#include <bits/stdc++.h>


using namespace std;

int main() {
    freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);

    int a,b,c,k;
    scanf("%d%d%d%d",&a,&b,&c,&k);

    int den = b-a+1;
    srand(time(0));
    int count = 0;
    for(int i=1;i<=1000000;i++){
        int check = a+rand()%(den);
       
        int ans = 0;
        do{
            if(check%10 == c){
                ans++;
            }
            check/=10;
        }while(check);
       
        
        if(ans>=k){
            count++;
        }
    }
    printf("%.4f\n",double(count)/1000000);
}