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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define st first
#define nd second
#define pb push_back
#define rep(i,a,b) for(ll i=a;i<=b;i++)
#define all(a) a.begin(),a.end()
const int mxN = 5e5+7;
int a[mxN];
int ms[mxN];
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n,k; cin >> n >> k;
	rep(i,1,n) cin >> a[i];
	set<int> ile_r;
	rep(i,1,n) ile_r.insert(a[i]);
	if(ile_r.size() < k){
		cout << "-1\n";
		return 0;
	}
	ll res = 0;
	int akt = 1;
	set<int> secik;
	for(int i=1;i<=n && akt <= k;i++){
		if(secik.find(a[i]) == secik.end()){
			res += i-akt;
			akt++;
			secik.insert(a[i]);
		}
	}
	cout << res << "\n";
}