#define LOCAL
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include<bits/stdc++.h>
using namespace std;
#define rep(i,x) for(ll i = 0; i < (ll)(x); i++)
#define rrep(i,x) for(ll i = (ll)(x)-1;0 <= i; i--)
#define reps(i,x) for(ll i = 1; i < (ll)(x)+1; i++)
#define rreps(i,x) for(ll i = (ll)(x); 1 <= i; i--)
#define all(x) (x).begin(), (x).end()
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> Pll;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<vvl> vvvl;
constexpr ll INF = numeric_limits<ll>::max()/4;
constexpr ll n_max = 2e5+10;
#define int ll

template <typename A, typename B>
string to_string(pair<A, B> p);
string to_string(const string &s) {return '"' + s + '"';}
string to_string(const char c) {return to_string((string) &c);}
string to_string(bool b) {return (b ? "true" : "false");}
template <size_t N>
string to_string(bitset<N> v){
    string res = "";
    for(size_t i = 0; i < N; i++) res += static_cast<char>('0' + v[i]);
    return res;
}
template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for(const auto &x : v) {
        if(!first) res += ", ";
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}
template <typename A, typename B>
string to_string(pair<A, B> p){return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";}

void debug_out() {cerr << endl;}
template<typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

signed main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    ll n; cin >> n;
    vector<ll> a(n);
    rep(i,n) cin >> a[i];
    vector<ll> cnt(n+1);
    cnt[a[0]]++;
    reps(i,n-1){
        if(a[i] != a[i-1])cnt[a[i]]++;
    }
    debug(cnt);
    if(*max_element(all(cnt)) == 1){
        cout << 0 << endl;
        return 0;
    }
    if(*max_element(all(cnt)) > 2){
        cout << -1 << endl;
        return 0;
    }
    ll c = 0;
    rep(i,n)if(cnt[i] == 2)c++;
    if(c > 1){
        cout << -1 << endl;
        return 0;
    }
    if(c == 1 && cnt[a[0]] == 2 && a[n-1] == a[0]){
        cout << 1 << endl;
        return 0;
    }
    else{
        cout << -1 << endl;
        return 0;
    }


}