#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;

int main(){
    int n;
    cin >> n;
    vector<int> a(n);
    vector<int> color(n);
    rep(i,n) cin >> a[i];
    rep(i,n) --a[i];
    rep(i,n){
        color[a[i]]++;
    }
    if(color[a[0]]==n){
        cout << 0 << endl;
        return 0;
    }
    int p=n-1;
    while(a[p]==a[0]){
        a.pop_back();
        --p;
        color[a[0]]--;
    }

    int cnt = 1;
    bool ok = true;
    rep(i,a.size()-1){
        if(a[i]==a[i+1]){
            ++ cnt;
        }else{
            if(color[a[i]]!=cnt) ok = false;
            cnt = 1;
        }
    }

    if(p==n-1 && ok) cout << 0 << endl;
    else cout << (ok?1:-1) << endl;
    return 0;
}