#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <vector>
#include <algorithm>
using namespace std;
int a, b, n;
long long k;
long long c[64][64];
int main(void) {
c[0][0] = 1;
for (int i = 1; i < 64; i++) {
c[i][0] = 1;
for (int j = 1; j <= i; j++) {
c[i][j] = c[i-1][j-1] + c[i-1][j];
}
}
scanf("%d %d %d %lld", &a, &b, &n, &k);
double area = log((double) a * b / 2.);
if (a == b) {
printf("%.10lf\n", (double) n * log(0.5) + area);
} else {
if (a < b) swap(a, b);
long long rank_sum = 0;
double ans = 0.0;
double ra = (double)a * a / (a * a + b * b);
double rb = (double)b * b / (a * a + b * b);
for (int i = 0; i <= n; i++) {
rank_sum += c[n][i];
if (rank_sum >= k) {
ans = (double) (n-i) * log(ra) + i * log(rb) + area;
break;
}
}
printf("%.10lf\n", ans);
}
return 0;
}
