struct Scanner {
    private var stack = [String]()
    private var index = 0
    
    mutating func peek() -> String {
        if stack.count == index {
            stack += readLine()!.split(separator: " ").map{ String($0) }
        }
        return stack[index]
    }
    
    mutating func next() -> String {
        let value = peek()
        index += 1
        return value
    }
    
    mutating func nextInt() -> Int {
        return Int(next())!
    }
    
    mutating func nextDouble() -> Double {
        return Double(next())!
    }
}

var scanner = Scanner()

let N = scanner.nextInt()
let T = (0 ..< N).map { _ in scanner.nextInt() }

func check(_ k: Int) -> Bool {
    let list = [0, 2, 4, 5, 7, 9, 11].map { ($0 + k) % 12 }
    return T.allSatisfy { list.contains($0) }
}

let candidates = (0 ..< 12).filter(check)
if candidates.count == 1 {
    print(candidates.first!)
} else {
    print("-1")
}