#include <iostream>
#include <cstdio>
#include <cmath>
#include <deque>
#include <algorithm>
using namespace std;
deque<double> q;
int a, b, n, k;
double dev;
void cut() {
deque<double> temp;
for (auto d : q) {
temp.push_back(d*a*a/dev/dev);
temp.push_back(d*b*b/dev/dev);
}
q.swap(temp);
}
int main() {
cin >> a >> b >> n >> k;
dev = sqrt(a*a + b*b);
q.push_back((double)a*b/2);
while (n--) {
cut();
}
sort(q.begin(), q.end());
auto r = q.begin();
advance(r, k);
printf("%0.10f\n", log(*r));
return 0;
}
