#include <bits/stdc++.h>
using namespace std;
#define int long long
#define FASTIO ios_base::sync_with_stdio(false); cin.tie(NULL);
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define pii pair<int, int>
#define ff first
#define ss second
#define PI acos(-1)
#define ld long double
template<class T> bool chmin(T& a, const T& b) {return a>b? a=b, true:false;}
template<class T> bool chmax(T& a, const T& b) {return a<b? a=b, true:false;}
const int mod = 1e9+7, N = 2e5+5;
int msb(int val){return sizeof(int)*8-__builtin_clzll(val)-1;}
int a[N], n, m, k;
const ld eps = 1e-9;
void solve(int test_case){
	cout << fixed << setprecision(10);
	int i, j;
	int area;
	ld bp, bq, w, ap,cq;
	cin >> area >> ap >> cq >> w;
	ld ans = ((ld)area - ap*cq)/2;
	cout << ans << '\n';
	return;
}

signed main(){
	FASTIO;
//~ #define MULTITEST 1
#if MULTITEST
	int _T;
	cin >> _T;
	for(int T_CASE = 1; T_CASE <= _T; T_CASE++)
		solve(T_CASE);
#else
	solve(1);
#endif
	return 0;
}