#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int x, y, z, w; cin >> x >> y >> z >> w;
  double lb = 0, ub = 1. * x / z - y;
  REP(_, 80) {
    double a = (lb + ub) / 2;
    double l = sqrt(x / (a + y) * x / (a + y) + 1. * y * y);
    double r = sqrt((a + y) * (a + y) + 1. * z * z) + w;
    (l > r ? lb : ub) = a;
  }
  cout << x - (lb * (x / (lb + y) - z) + (lb + y) * z + x / (lb + y) * y) / 2 << '\n';
  return 0;
}