#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3fLL;
const double EPS = 1e-8;
const int MOD = 1000000007;
// const int MOD = 998244353;
const int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
const int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n; cin >> n;
  vector<int> a(n); REP(i, n) cin >> a[i], --a[i];
  vector<vector<int>> idx(n);
  REP(i, n) idx[a[i]].emplace_back(i);
  int cnt = 0;
  REP(i, n) cnt += idx[i].empty() || idx[i].back() - idx[i].front() + 1 == idx[i].size();
  if (cnt == n) {
    cout << "0\n";
    return 0;
  }
  if (cnt < n - 1) {
    cout << "-1\n";
    return 0;
  }
  int l = 0;
  while (a[l] == a.back()) ++l;
  rotate(a.begin(), a.begin() + l, a.end());
  REP(i, n) idx[i].clear();
  REP(i, n) idx[a[i]].emplace_back(i);
  cnt = 0;
  REP(i, n) cnt += idx[i].empty() || idx[i].back() - idx[i].front() + 1 == idx[i].size();
  cout << (cnt == n ? 1 : -1) << '\n';
  return 0;
}