#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main()
{
    int sol,A,B,C,K,cnt,x,Timp;
 
    freopen("cifre.in","r",stdin);
 
    scanf("%d%d%d%d",&A,&B,&C,&K);
 
    Timp=1005000;
 
    srand(time(NULL));
 
    sol=0;
    while(Timp)
    {
        x=A+rand()%(B-A+1);
 
        cnt=0;
        while(x)
        {
            if(x%10==C) cnt++;
            x/=10;
        }
 
        if(cnt>=K) sol++;
 
        Timp--;
    }
 
    freopen("cifre.out","w",stdout);
 
    printf("%.4f",(double)sol/1005000);
 
    return 0;
}