#include <bits/stdc++.h>
using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
//typedef vector<vector<ll>> Graph;

const ll mod = 1e9 + 7;
//const ll mod = 998244353;

#define REP(i,n) for(ll i=0;i<(ll)n;i++)
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define spa << " " <<
#define fi first
#define se second

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){
os << "(" << v.first << ", " << v.second << ")"; return os;
}
template<class T> ostream& operator << (ostream& os, const vector<T> v){
for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << " ";} os << v[i];} return os;
}
template<class T> ostream& operator << (ostream& os, const vector<vector<T>> v){
for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << endl;} os << v[i];} return os;
}

template<typename T> void debug(vector<vector<T>>&v,ll h,ll w){for(ll i=0;i<h;i++)
{cerr<<v[i][0];for(ll j=1;j<w;j++)cerr spa v[i][j];cerr<<endl;}};
void debug(vector<string>&v,ll h,ll w){for(ll i=0;i<h;i++)
{for(ll j=0;j<w;j++)cerr<<v[i][j];cerr<<endl;}};
template<typename T> void debug(vector<T>&v,ll n){if(n!=0)cerr<<v[0];
for(ll i=1;i<n;i++)cerr spa v[i];
cerr<<endl;};

string num2bit(ll num, ll len){
    string bit = "";
    REP(i, len){
    bit += char('0'+(num>>i & 1));
    }
    return bit;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N;
    cin >> N;
    vector<ll> A(N);
    REP(i, N) cin >> A[i];

    vector<ll> L;
    REP(i, N){
        int j = i;
        while(j<N and A[i]==A[j]) j++;
        L.push_back(A[i]);
        i = j-1;
    }

    if((int)L.size()==1){
        cout << 0 << endl;
    }else{
        map<ll, ll> mp;
        ll k = L.size();
        ll res = 0;
        if(L[0] == L[k-1]){
            res++;
            L.pop_back();
            k--;
        }
        bool ok = true;
        REP(i, k){
            if(mp[L[i]]==1) ok = false;
            mp[L[i]]++;
        }

        cout << (ok ? res : -1) << endl;
    }

    return 0;
}