#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
long long a, b, c, d, e, f;
cin >> a >> b >> c >> d >> e >> f;
vector<long long> t1 = {a, b, c};
vector<long long> t2 = {d, e, f};
sort(t1.begin(), t1.end());
sort(t2.begin(), t2.end());
if (t1 != t2) {
cout << -1;
return 0;
}
long long x = t1[0], y = t1[1], z = t1[2];
if (x + y <= z) {
cout << -1;
return 0;
}
if (!(x == y || y == z)) {
cout << -1;
return 0;
}
double p = (x + y + z) / 2.0;
double v = sqrt(p * (p - x) * (p - y) * (p - z));
double l = 2.0 * v;
cout << fixed << setprecision(2) << l;
return 0;
}
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