#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max() / 32 * 15 + 208;

int main() {
    double a, b, c;
    cin >> a >> b >> c;
    double x = (a*a+b*b-c*c)/(2*a*b);
    x = sqrt(1-x*x);
    printf("%.10lf\n", a*b*x/8);
    return 0;
}