#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;
int N;
int A[100000];
set<int> st;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    cin >> N;
    int cnt = 0;
    int prev = -1;
    for(int i = 0; i < N; i++){
        cin >> A[i];
        if(prev != A[i]) {
            cnt++;
            prev = A[i];
        }
        st.insert(A[i]);
    }
    if(st.size() == cnt){
        cout << 0 << endl;
    }else if(st.size() == cnt-1){
        if(A[0] == A[N-1]) cout << 1 << endl;
        else cout << -1 << endl;
    }else{
        cout << -1 << endl;
    }
}