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#include <bits/stdc++.h>

using namespace std;
#define pb push_back
typedef long long ll;
const int MAX = 5e5 + 7;

bool vis[MAX];
int t[MAX];
vector <int> v;

ll solve(int n, int k, int lic){
	if(lic < k) return -1;
	
	ll ans = 0;
	sort(v.begin(), v.end());
	for(int i = 0; i < k; i++){
		ans += (ll)v[i] - (ll)i;
	}
	return ans;
}

int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	int n, k;
	cin >> n >> k;
	for(int i = 0; i < n; i++){
		cin >> t[i];
		if(!vis[t[i]]) {
			v.pb(i);
			vis[t[i]] = true;
		}
	}
	cout << solve(n, k, v.size());
	return 0;
}