#include <cmath>
#include <vector>
#include <iostream>
#include <algorithm>
#include <functional>
using namespace std;
const int inf = 1012345678;
int main() {
	int N;
	cin >> N;
	vector<int> A(N);
	for (int i = 0; i < N; ++i) {
		cin >> A[i]; --A[i];
	}
	vector<vector<int> > G(N);
	for (int i = 0; i < N; ++i) {
		G[A[i]].push_back(i);
	}
	int ans = 0;
	for (int i = 0; i < N; ++i) {
		if (G[i].empty()) continue;
		int l = inf, r = -inf;
		for (int j : G[i]) {
			l = min(l, j);
			r = max(r, j);
		}
		if (r - l + 1 != G[i].size()) {
			sort(G[i].begin(), G[i].end());
			bool f = true;
			for (int j = 0; j < G[i].size(); ++j) {
				if (G[i][j] != j && G[i][j] != N - (G[i].size() - j)) {
					f = false;
					break;
				}
			}
			if (f) ++ans;
			else {
				ans = -1;
				break;
			}
		}
	}
	cout << ans << endl;
	return 0;
}