#include <iostream>
#include <vector>
#include <set>
using namespace std;

vector<int> pos[500005];
set<pair<int,int>> bottles;

int main(){
	int n, k;
	cin >> n >> k;
	for (int i = 0; i < n; i++){
		int a;
		cin >> a;
		bottles.insert(make_pair(i, a));
		pos[a].push_back(i);
	}
	long long res = 0;
	int sat = 0;
	for (int i = 0; i < k; i++){
		if (bottles.empty()){
			cout << "-1" << "\n";
			return 0;
		}
		pair<int,int> c = *bottles.begin();
		res += c.first - i;
		for (int j = 0; j < pos[c.second].size(); j++){
			bottles.erase(make_pair(pos[c.second][j], c.second));
		}
	}
	cout << res << "\n";
}