#include <stdio.h>

int main(){

FILE *file1, *file2;
long a, b, i;
int c, k;
float fc = 0.0;

file1 = fopen("cifre.in", "r");
file2 = fopen("cifre.out", "w");

if(fscanf(file1, "%ld", &a))
if(fscanf(file1, "%ld", &b))
if(fscanf(file1, "%d", &c))
if(fscanf(file1, "%d", &k))

for(i=a; i<=b; i++){
int count = 0;
long n = i;
    while(n>0){
        if(n%10==c)
            count++;
        n /= 10;
    }
    if(count>=k)
        fc++;
}
fprintf(file2,"%.4f", fc/(b-a+1));

return 0;
}