import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		ArrayDeque<Integer> deq = new ArrayDeque<>();
		HashMap<Integer, ArrayList<Integer>> positions = new HashMap<>();
		int idx = 0;
		for (int i = 0; i < n; i++) {
		    int x = sc.nextInt();
		    if (deq.size() == 0 || deq.peekLast() != x) {
		        if (!positions.containsKey(x)) {
		            positions.put(x, new ArrayList<>());
		        }
		        positions.get(x).add(idx);
		        deq.add(x);
		        idx++;
		    }
		}
		int count = 0;
		for (ArrayList<Integer> list : positions.values()) {
		    if (list.size() <= 1) {
		        continue;
		    }
		    if (list.size() == 2 && list.get(0) == 0 && list.get(1) == idx - 1) {
		        count++;
		        continue;
		    }
		    System.out.println(-1);
		    return;
		}
		System.out.println(count);
	}
}