/**
 *   @FileName	f.cpp
 *   @Author	kanpurin
 *   @Created	2020.05.15 23:34:07
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int n;
    cin >> n;
    vector< int > a(n);
    vector< int > used(n, -1);
    bool same = true;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        if (i > 0 && a[0] != a[i]) {
            same = false;
        }
    }
    if (same) {
        cout << 0 << endl;
        return 0;
    }
    int ok = -1;
    for (int i = 1; i < n; i++) {
        if (ok == -1 && a[i] != a[0]) {
            ok = i;
            break;
        }
    }
    used[a[0]-1] = 1;
    for (int i = ok; i < n; i++) {
        if (a[i] != a[i-1] && abs(used[a[i]-1]) == 1) {
            used[a[i]-1] = 0;
        }
        else if (a[i] != a[i-1]) {
            puts("-1");
            return 0;
        }
    }
    if (a[0] == a[a.size()-1]) {
        cout << 1 << endl;
    }
    else if (used[a[0]-1] == 0) {
        cout << -1 << endl;
    }
    else {
        cout << 0 << endl;
    }
    return 0;
}