func cal(X: [Int], Y: [Int64]) -> Double {
    let total = Double(Y.reduce(0, +))
    var z = zip(X, Y).map { pow(Double($0), 2) * Double($1) }.reduce(0, +) / Double(total)
    return z
}

func cal(xs: [Int], ys: [Int64]) -> Double {
    let xs = xs.lazy.map(Double.init)
    let ys = ys.lazy.map(Double.init)

    let total = ys.reduce(0, +)
    return zip(xs, ys).map { $0*$0 * $1 }.reduce(0, +) / total
}

extension Sequence where Element: Numeric {
    func sum() -> Element {
        return reduce(0, +)
    }
}

func cal(xs: [Int], ys: [Int64]) -> Double {
    let xs = xs.lazy.map(Double.init)
    let ys = ys.lazy.map(Double.init)

    let total = ys.sum()
    return zip(xs, ys).map { $0*$0 * $1 }.sum() / total
}