#include<stdio.h>

#define input "cifre.in"
#define output "cifre.out"

long verifica(long n);

long a, b, c, k;

int main()
{
    FILE *in, *out;

    in = fopen (input, "r");
    out = fopen (output, "w");

    long cont=0, i;

    fscanf(in, "%ld%ld%ld%ld", &a, &b, &c, &k);

    for(i=a; i<=b; ++i)

        if(verifica(i))

            ++ cont;

    float rez = (float) cont/(b-a+1) ;

    fprintf(out, "%.4f", rez);

    return 0;

}

long verifica(long n)
{
    long ind = 0;

    while(n)
    {
        if(n%10==c)

            ++ind;

        n/=10;

        if(ind == k)

            return 1;

    }

    return 0;

}