// clang-format off
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <iomanip>
#include <bitset>
#include <vector>
#include <algorithm>
#include <random>
#include <map>
#include <string>
#include <set>
#include <deque>
#include <cassert>


const int N = 2e5 + 7, A = 26, C = 2, MOD = 998244353;
const long long INF = 1e18;
using namespace std;
using ll = long long;
using ld = long double;

const ld EPS = 1e-9;

long double w1, h1, w2, h2;
long double ans = 1e18;
void process() {
    if (h1 <= h2) {
        if (w2 <= w1) {
            ans = min(ans, (w1 - w2) * h1 / 2);
        }
        else {
            ans = 0;
        }
    }
    else {
        if (w2 >= w1) {
            ans = min(ans, (h1 - h2) * w1 / 2);
        }
        else {
            ans = min(ans, (h1 * w1) - h2 * w2);
        }
    }
}

void solve() {
    cin >> w1 >> h1 >> w2 >> h2;

    process();
    swap(h1, w1);
    process();
    swap(h2, w2);
    process();
    swap(h1, w1);
    process();
    cout <<fixed <<  setprecision(1) << ans;
}

signed main() {
#ifdef _DEBUG
    freopen("input.txt", "r", stdin);
#else
    std::ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    int t;
    t = 1;
    while (t--) {
        solve();
    }
}