def main():
    A, B, X, Y = map(int, input().split())

    needed_tea_size_for_coffee_base = X * (B / A)
    if needed_tea_size_for_coffee_base <= Y:
        print(X + needed_tea_size_for_coffee_base)
    else:
        needed_coffee_size_for_tee_base = Y * (A / B)
        print(Y + needed_coffee_size_for_tee_base)

if __name__ == '__main__':
    main()