#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0; i<(n); i++)
using ll = long long;
using P = pair<int,int>;
int visited[110000];
int main()
{
    int n;cin >>n ;
    int a[n+1];
    rep(i,n)cin >> a[i];
    vector<int> c;
    int now = a[0];
    rep(i,n-1){
        if(a[i+1] != now){
            c.push_back(now);
            now = a[i+1];
        }


    }
    c.push_back(now);
    bool ok = true;
    int m = c.size();
    
    if(m <= 2){
        cout << 0 << endl;
        return 0;
    }
    
    int flag = 0;
  int ng;
    if(c[0] == c[m-1]){
      ng = c[0];
        c.pop_back();
        reverse(c.begin(),c.end());
        c.pop_back();
        flag = 1;
      

    }
  visited[ng] = 1;
    for(auto p:c){
        
        
        if(visited[p])ok = false;
        visited[p] = 1;
    }
    if(ok){
        if(flag)cout << 1 << endl;
        else cout << 0 << endl;


    }
    else{

        cout << -1 << endl;
    }


    return 0;
}