#include <stdio.h>

int main(){

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

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

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

pc = b - a + 1;

for(i=a; i<=b; i++){
    count = 0;
    n = i;
    while(n>0){
        if(n%10==c)
            count++;
        n /= 10;
    }
    if(count>=k)
        fc++;
}

    p = fc / pc;
    fprintf(file2, "%.4f", p);

}