#include <bits/stdc++.h>
//#include <boost/multiprecision/cpp_int.hpp>
using namespace std;
//g++ hoge.cpp (-std=c++17) -I . で実行
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
typedef long long ll;
typedef pair<ll,ll> prl;
typedef vector<ll> vcl;
typedef map<ll,ll> mapl;
typedef unordered_map<ll,ll> umap;
#define pb push_back
#define all(v) v.begin(), v.end()
#define rep(i,a,b) for(ll i=a;i<=b;i++)
#define repi(i,a,b) for(int i=a;i<=b;i++)
#define repr(i,a,b) for(ll i=a;i>=b;i--)
#define reps(i,v) for(ll i=0;i<v.size();i++)
#define rept(itr,v) for(auto itr=v.begin();itr!=v.end();itr++)
#define rept1(itr,v) for(auto itr=v.begin()+1;itr!=v.end();itr++)
#define rept2(itr,v) for(auto itr=v.begin();itr!=v.end()-1;itr++)
#define print(x) cout << (x) << endl;
#define fprint(x) printf("%.15lf\n",(x));
#define print2(x,y) cout << (x) << " " << (y) << endl;
#define YesNo(test) cout << ((test) ? "Yes" : "No") << endl
#define debug cout << "debug_print" << endl
#define Bint boost::multiprecision::cpp_int
template<typename T> void chmin(T &a, const T &b) { a = min(a, b); }
template<typename T> void chmax(T &a, const T &b) { a = max(a, b); }
ll myceil(ll a, ll b) { return (a+b-1) / b; }
//const ll mod = 1e9+7;
//const ll mod = 998244353;
//typedef modint1000000007 mint;
//typedef modint998244353 mint;
//typedef modint mint
//cout << sum.val() << endl;
//mint::set_mod(mod); modが固定でないとき

double x,y,z,w;

int main() {
    // your code goes here
    cin >> x >> y >> z >> w;
    fprint(x/2.0-(y*z)/2.0);
    return 0;
}