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#include <bits/stdc++.h>
using namespace std;
 
typedef unsigned long long ull;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> PII;
typedef pair<ll, int> PILL;
typedef pair<ll, ll> PLL;
 
const int MAX_N = 5e5+5;
const int M = 3e4+5;
const ll INF = (ll)(1e18);
const int inf = 1e9;
const ll MOD = 1000000007LL;

int n, k;
int a[MAX_N];
bool appeared[MAX_N];

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
			
	cin >> n >> k;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	
	ll ans = 0LL;
	int now = 0;
	
	for (int i = 0; i < n; i++) {
		if (now >= k) break;
		
		if (!appeared[a[i]]) {
			ans += i - now;
			now++;
			appeared[a[i]] = true;
		}
	}
	
	if (now < k) ans = -1LL;
	
	cout << ans << '\n';
	
    return 0;
}