#include <iostream>
#include <queue>

using namespace std;

int main() {

	int n, k;
	cin >> n;
	cin >> k;

	int tab[n];
	for (int i = 0; i < n; i++) {
		tab[i] = 0;
	}

	int bad = 0, a;
	queue<int> q;

	for (int i = 0; i < k; i++) {
		cin >> a;
		
		if (tab[a] != 0) {
			q.push(i);
			bad++;
		}
		tab[a]++;
	}

	int switches = 0;
	for (int i = k; i < n; i++) {
		cin >> a;

		if (tab[a] == 0 && !q.empty()) {
			switches += i - q.front();
			q.pop();
		}
		tab[a]++;
	}

	if (!q.empty()) {
		cout << -1 << endl;
	} else {
		cout << switches << endl;
	}

	return 0;
}