#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <math.h>
long long combination(int n, int r){
int p[40] = { 0 };
for (int i = r + 1; i <= n; i++){
int t = i;
for (int j = 2; j <= t; j++){
while (t % j == 0){
t /= j;
p[j]++;
}
}
}
for (int i = 1; i <= n - r; i++){
int t = i;
for (int j = 2; j <= t; j++){
while (t % j == 0){
t /= j;
p[j]--;
}
}
}
long long x = 1;
for (int i = 2; i < 40; i++)
while (p[i]--)
x *= i;
return x;
}
int main(){
int a, b, N;
long long K;
scanf("%d %d %d %lld", &a, &b, &N, &K);
double A = double(a < b ? a : b);
double B = double(a > b ? a : b);
double C = hypot(A, B);
long long p = 0;
for (int i = 0; i < K; i++){
p += combination(N, i);
if (p >= K){
double x = A * B / 2;
for (int j = 0; j < N - i; j++)
x *= B * B;
for (int j = 0; j < i; j++)
x *= A * A;
for (int j = 0; j < N; j++)
x /= C * C;
printf("%.12lf", log(x));
break;
}
}
return 0;
}
