#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <numeric>
#include <bitset>
#include <cmath>

static const int MOD = 1000000007;
using ll = long long;
using u32 = unsigned;
using u64 = unsigned long long;
using namespace std;

template<class T> constexpr T INF = ::numeric_limits<T>::max()/32*15+208;

int main() {
    int n;
    cin >> n;
    vector<int> v(n);
    for (auto &&i : v) scanf("%d", &i), i--;
    v.erase(unique(v.begin(), v.end()), v.end());
    vector<int> cnt(n);
    int a = 0;
    for (auto &&i : v) {
        cnt[i]++;
        if(cnt[i] == 2) a++;
        if(cnt[i] == 3) return puts("-1"), 0;
    }
    if(a == 0) puts("0");
    else if(v.back() == v.front()) puts("1");
    else puts("-1");
    return 0;
}