#include<cstdio>
FILE *in,*out;
using namespace std;
int i, a, b, m, c, k;
float n, v;
int j;
int main()
{
    in=fopen("cifre.in", "rt");
    fscanf(in,"%d %d %d %d", &a, &b, &c, &k);
    fclose(in);
    for(i=a; i<=b; i++)
    {
        m=i;
        j=0;
        while(m!=0)
        {
            if(m%10==c)
                j++;
            m/=10;
        }
        if(j>=k)
            n++;
        v++;
    }
    out=fopen("cifre.out","wt");
    fprintf(out,"%1.4f", n/v);
    fclose(out);
    return 0;
}