#include <bits/stdc++.h>
using namespace std;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

int main() {
    int x, y, z, w; cin >> x >> y >> z >> w;
    cout << fixed << setprecision(8) << (x - (x + y * z) / 2.0) << endl;
}