import java.io.BufferedReader
import java.io.InputStream
import java.io.InputStreamReader
import java.io.PrintWriter
import java.lang.StringBuilder
import java.util.*

fun PrintWriter.solve(sc: FastScanner) {
    val n = sc.nextInt()
    val a = Array(n) { sc.nextInt() - 1 }
    val segcount = Array(n) { 0 }
    val num = a.distinct().count()
    var color = a[0]
    for (i in 0 until n) {
        if (color != a[i]) {
            segcount[color]++
            color = a[i]
        }
        if (i == n - 1 && color != a[0]) {
            segcount[color]++
        }
    }
    if (segcount.any { it >= 2 }) {
        println(-1)
    } else if (a[0] == a[n - 1] && num >= 2) {
        println(1)
    } else {
        println(0)
    }
}

fun main() {
    val writer = PrintWriter(System.out, false)
    writer.solve(FastScanner(System.`in`))
    writer.flush()
}

class FastScanner(s: InputStream) {
    private var st = StringTokenizer("")
    private val br = BufferedReader(InputStreamReader(s))

    fun next(): String {
        while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine())

        return st.nextToken()
    }

    fun nextInt() = next().toInt()
    fun nextLong() = next().toLong()
    fun nextLine() = br.readLine()
    fun nextDouble() = next().toDouble()
    fun ready() = br.ready()
}