answer[3]

answer[3].swift

#!/usr/bin/env xcrun --sdk macosx swift

/// Run: swift answer\[3\].swift

import Foundation

enum CustomError: Error {
    case msg(String)
}

func calculateSeries(_ n: Int) throws -> Double {
    var cur: Double = 0
    
    if n < 2 {
        throw CustomError.msg("Invalid input")
    }

    for i in 2...n {
        cur += 1 / (Double(i) * Double(i - 1))
    }
    return cur
}

func test() {
    let n = 2
    
    do {
        let result = try calculateSeries(n)
        print(result)
    } catch {
        print(error.localizedDescription)
    }
}

test()