#include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; typedef long long ll; typedef pair pi; typedef pair pl; int main() { double xa, ya, xb, yb; cin >> xa >> ya >> xb >> yb; double a = abs(yb -ya) / abs(xb + xa); double b = yb - xb*a; printf("%.10f\n",b); return 0; }