#include <stdio.h>
#include <stdlib.h>
#include <time.h>

FILE *fin, *fout;
int c, k;
#define Q 1000121
inline bool nr(int x){
    short int ans=0;
    do{
        ans+=(x%10==c);
        x/=10;
    }while(x);
    return ans>=k;
}

int main()  {
    srand(time(0));
    fin = fopen("cifre.in", "r");
    fout = fopen("cifre.out", "w");
    int a, b;
    fscanf(fin, "%d%d%d%d", &a, &b, &c, &k);
    double ans;
    if(b-a+1<=Q){
        int s=0;
        for(int i=a; i<=b; i++)
            s+=nr(i);
        ans=s/(double)(b-a+1);
    }else{
        int s=0;
        for(int i=0; i<Q; i++)
            s+=nr(a+rand()%(b-a+1));
        ans=s/(double)Q;
    }

    fprintf(fout, "%.4lf", ans);
    fclose(fin);
    fclose(fout);
    return 0;
}