#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  cin.tie(nullptr)->sync_with_stdio(false);
  int x, y, z, w;
  cin >> x >> y >> z >> w;
  cout << fixed << setprecision(10) << (x - y * z) / 2.0 << '\n';
  return 0;
}