#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
template<class T> bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T> bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1e9+7;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int main(){
    int n;
    scanf("%d", &n);
    int a[n];
    rep(i, n) scanf("%d", &a[i]), --a[i];
    vector<pii> p[n];
    for(int l=0; l<n;){
        int r = l;
        while(r < n && a[l] == a[r]) ++r;
        p[a[l]].emplace_back(l, r);
        l = r;
    }
    bool flag = false;
    rep(i, n){
        if(p[i].size() >= 3 || (p[i].size() == 2 && flag)){
            puts("-1");
            return 0;
        }
        else if(p[i].size() == 2) flag = true;
    }

    if(!flag) printf("%d\n", 0);
    else{
        rep(i, n)if(p[i].size() == 2){
            if(p[i][0].fi == 0 && p[i][1].se == n){
                printf("%d\n", 1);
                return 0;
            }
        }
        puts("-1");
    }
}