#include <stdio.h>
#include <algorithm>
#include <vector>
using namespace std;

vector<int> wow[1001001];
int N, A[100100];

int main()
{
	scanf ("%d", &N);
	for (int i = 0; i < N; i++){
		scanf ("%d", &A[i]);
		wow[A[i]].push_back(i);
	}
	bool g = 1; int r = 0;
	for (int i = 1; i <= N; i++) if (!wow[i].empty()){
		sort(wow[i].begin(), wow[i].end());
		if (wow[i].size() == wow[i].back() - wow[i][0] + 1) continue;
		bool e = 0;
		for (int j = 1; j < wow[i].size(); j++){
			if (wow[i].size() == (wow[i][j - 1] - wow[i][0] + 1) + (wow[i].back() - wow[i][j] + 1)) e = 1;
		}
		if (!e) g = 0;
		else r++;
	}
	if (!g || r > 1) puts("-1");
	else if (r) puts("1");
	else puts("0");

	return 0;
}