#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N; cin >> N;
    vector<int> A(N);
    rep(i,N) cin >> A[i];
    vector<pair<int,int>> V = {{A[0], 1}};
    rep(i,N) if(i) {
        if(V.back().first == A[i]) V.back().second++;
        else V.push_back({A[i], 1});
    }
    
    int cost = 0;
    if(V.size() >= 2 && V[0].first == V.back().first) {
        cost += min(V[0].second, V.back().second);
        V[0].second += V.back().second;
        V.pop_back();
    }
    vector<int> used(N + 1, 0);
    for(auto [x, cnt] : V) {
        if(used[x]){ cout << -1 << endl; return 0; }
        used[x] = 1;
    }

    cout << cost << endl;
}