#include <iostream>
#include <iomanip>

using ldouble = long double;

void solve() {
    ldouble x1, y1, x2, y2;
    std::cin >> x1 >> y1 >> x2 >> y2;

    std::cout << std::fixed << std::setprecision(10)
              << (x1 * y2 + x2 * y1) / (x1 + x2)
              << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}