#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T, class U> using Pa = pair<T, U>;
template <class T> using vec = vector<T>;
template <class T> using vvec = vector<vec<T>>;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N;
    cin >> N;
    vec<int> A(2*N);
    vec<set<int>> s(N);
    for(int i=0;i<N;i++){
        cin >> A[i];
        A[i]--;
        s[A[i]].insert(i);
        A[i+N] = A[i];
    }
    auto check = [&](int a){
        if(s[a].empty()) return true;
        int l = *s[a].begin(),r = *s[a].rbegin();
        return r-l+1==s[a].size();
    };
    int cnt = 0;
    vec<int> B(N);
    for(int i=0;i<N;i++){
        bool a = check(i);
        B[i] += a;
        cnt += a;
    }
    if(cnt==N){
        cout << 0 << "\n";
        return 0;
    }
    bool ok = false;
    for(int i=N;i<2*N;i++){
        int a = A[i-N];
//        cerr << a << " " << B[a] << "\n";
        s[a].erase(i-N);
        cnt -= B[a];
        B[a] = 0;
        s[a].insert(i);
        B[a] = check(a);
        cnt += B[a];
        ok |= cnt==N;
    }
    cout << (ok? 1:-1) << "\n";
}