#include #include #include #include #include #include #include #include #include static const int MOD = 1000000007; using ll = int64_t; using u32 = uint32_t; using namespace std; template constexpr T INF = ::numeric_limits::max()/32*15+208; int main() { long double a, b, c, d; cin >> a >> b >> c >> d; cout << setprecision(15) << c*d/(a+b) << "\n"; return 0; }