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#include <bits/stdc++.h>

#define endl '\n'
#define LL long long
#define fi first
#define sc second
using namespace std;

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	
	int N, K;
	cin >> N >> K;
	long long Result = 0;
	int d = 0;
	vector<bool> Seen(N);
	for(int i = 0; i < N; i++){
		int a;
		cin >> a; a--;
		if(i < K){
			if(Seen[a]){
				Result -= i; //Result += K - i;
				d++;
			}
			else{
				Seen[a] = true;
			}
		}
		else{
			if(!Seen[a] && d != 0){
				Seen[a] = true;
				Result += i; //Result += i - K;
				d--;
			}
		}
	}
	if(d != 0){
		cout << -1 << endl;
	}
	else{
		cout << Result << endl;
	}
	return 0;
}