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#include <bits/stdc++.h>
#define ll long long

using namespace std;

int main(){
	ios_base::sync_with_stdio(0);
	ll n, k; cin>>n>>k;
	vector<bool> tab(n+10);

	ll pt = 0;
	ll wyn = 0;
	for(ll i = 0; i < n; i++){
		ll a; cin>>a;
		if(pt==k) continue;
		if(!tab[a]){
			tab[a] = 1;	
			wyn+=(i-pt);
			pt++;
		}
	}
	if(pt!=k) cout<<-1<<'\n';
	else cout<<wyn<<'\n';
	return 0;
}