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#include <bits/stdc++.h>
 
#define st first
#define nd second
#define pb push_back
#define BOOST ios_base::sync_with_stdio(0); cout.tie(0); cin.tie(0);
#define PI 3.14159265359
 
using namespace std;
 
typedef long long ll;
constexpr ll MOD = 1000000007, sup = 29;
constexpr ll MOD_1 = 1234567891, sup_1 = 1009;
constexpr ll MOD_2 = 1000000009, sup_2 = 107;
constexpr int MXN = 500000+10, CZAPA = (1<<19)/*, INF = 1000000000*/;
constexpr ll INF = 1000000000000000000;

int tab[MXN], n, k;
bool vst[MXN];
vector<ll> V;

int main(){
	BOOST;
	
	cin>> n >> k;
	for(int i=1; i<=n; i++){
		cin>> tab[i];
		if(!vst[tab[i]] && (V.size()) < k){
			V.pb(ll(i));
			vst[tab[i]] = true;
		}
	}
	if(int(V.size()) < k){
		cout<< "-1\n";
		return 0;
	}
	ll ans = 0, cnt = 1;
	for(ll i : V){
		ans += i - cnt;
		cnt++;
	}
	cout<< ans << '\n';

	return 0;
}