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#include <iostream>
#include <unordered_set>

using namespace std;

#define N 500005

int n, k;
int a[N];
unordered_set<int> us;

long long ans;

int main(){
	cin.tie(0);
	ios_base::sync_with_stdio(0);

	cin >> n >> k;

	for(int i=0;i<n;i++){
		cin >> a[i];
	}

	for(int i=0;i<n;i++){
		if(k == 0) break;
		else if(us.count(a[i]) == 0){
			ans += i - us.size();
			us.insert(a[i]);
			k--;
		}
	}

	if(k > 0) cout << -1;
	else cout << ans;

	return 0;
}