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#include <cstdio>
#include <iostream>
#include <algorithm>
#include <string>
#include <unordered_set>
#include <queue>

using namespace std;
int a[500000+1];

int main(){
	long long int k = 0;
	long long int n = 0;
	unordered_set<int> oren;

	cin >> n;
	cin >> k;
	for(int i = 0; i < n; ++i){
		cin >> a[i];
	}
	
	long long int wynik = 0;
	long long int place=0;
	for(int i = 0; i < n && place < k; ++i){
		if(oren.find(a[i]) == oren.end()){
			oren.insert(a[i]);
			wynik+=i-place;
			place++;
		}
	}
	
	if(oren.size() != k){
		cout << -1 << "\n";	
	} else {
		cout << wynik << "\n";
	}
	
	
	return 0;
}