#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
using namespace std;

const int maxRand = 600000;

int A, B, C, K, ans, val;


int CifreC(int N)
{
    int cnt = 0;
    while(N)
    {
            if(N % 10 == C) cnt ++;
            N /= 10;
    }
    return (cnt >= K);
}


int main()
{
    freopen("cifre.in", "r", stdin);
    freopen("cifre.out", "w", stdout);
    srand(time(NULL));
    int i;
    scanf("%i %i %i %i", &A, &B, &C, &K);
    if(B - A <= maxRand)
    {
         ans = 0;
         for(i = A; i <= B; i++)
               if(CifreC(i))
                            ans ++;
         printf("%.4lf\n", 1.0 * ans / (B - A + 1));
    }else
    {
         ans = 0;
         i = maxRand;
         while(i)
         {
               val = (rand() % (B - A + 1)) + A;
               if(CifreC(val))
                              ans ++;
               i --;
         }
         printf("%.4lf\n", 1.0 * ans / maxRand);
    }
    return 0;
}