#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define REP1(i, n) for(int i=1; i<=(n); i++)
#define RREP1(i, n) for(int i=(n); i>=1; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define out(...) printf(__VA_ARGS__)

int dxy[] = {0, 1, 0, -1, 0};

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/
#if DEBUG
#define SIZE 100
#else
#define SIZE 123450
#endif

int N,A[SIZE],C[SIZE];
map<int,vector<pair<int,int>>> M;

void solve() {
    cin>>N;
    int from = 0;
    REP(i,N) {
        cin>>A[i];
        if (i && A[i] != A[i-1]) {
            int a = A[i-1];
            M[a].push_back({from,i});
            from = i;
        }
    }
    M[A[N-1]].push_back({from,N});
    int ans = 0;
    for (auto p:M) {
        if (p.first == A[0] && p.second.size() == 2 && p.second.back().second==N) {
            ans = 1;
        }
        else if (p.second.size() > 1) {
            ans = -1;
            break;
        }
    }
    cout << ans << endl;
}