#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

bool same(const vector<int> &a) {
  REP (i, a.size())
    if (a[i] != a[0])
      return false;
  return true;
}

int solve(vector<int> &a) {
  if (same(a))
    return 0;

  int ret = 0;
  if (a.front() == a.back()) {
    ret++;
    while (a.front() == a.back()) {
      a.pop_back();
    }
  }
  set<int> vis;
  for (int i = 0; i < a.size(); ) {
    if (vis.count(a[i]))
      return -1;
    vis.insert(a[i]);
    int j = i;
    while (j < a.size() && a[j] == a[i])
      ++j;
    i = j;
  }
  return ret;
}

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

  int n;
  cin >> n;
  vector<int> a(n);
  REP (i, n) cin >> a[i];
  cout << solve(a) << endl;
  
  return 0;
}