# include <stdlib.h>
# include <cstdio>

using namespace std;

int uc, c, k, a, b, ct;
double ct1;
int nrapar(int val)
{
    ct = 0;
    while (val)
    {
          uc = val % 10;
          if (uc == c) ct++;
          if (ct >= k) break;
          val = val / 10;
          }
    return ct;
}
    
double rez;
int main()
{int i;
    freopen("cifre.in","r",stdin);
    freopen("cifre.out","w",stdout);
    
    scanf("%d%d%d%d",&a,&b,&c,&k);
    
    for (i = a; i <= b; i++)
        if (nrapar(i) >= k) ct1++;
    
    
    rez = ct1/(b-a+1);
    printf("%.4lf", rez);
    
return 0;
}