#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    int n;
    cin >> n;
    int a[n];
    rep(i,n)cin >> a[i];

    int f[MAX_N] = {};
    f[a[0]] = 1;
    srep(i,1,n){
        if(a[i] != a[i-1]){
            f[a[i]]++;
        }
    }

    int ma = 0;
    rep(i,MAX_N){
        ma = max(ma, f[i]);
    }
    if(ma > 2){
        cout << -1 << endl;
        return 0;
    }else if(ma == 1){
        cout << 0 << endl;
        return 0;
    }else{
        int cnt = 0;
        int arg = -1;
        rep(i,MAX_N){
            if(f[i] == 2){
                cnt++;
                arg = i;
            }
        }
        if(cnt > 1 || arg != a[0] || arg != a[n-1]){
            cout << -1 << endl;
        }else{
            cout << 1 << endl;
        }
    }

    return 0;
}