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#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define rep(a, b) for(ll a = 0; a < (b); ++a)
#define st first
#define nd second
#define pb push_back
#define all(a) a.begin(), a.end()
const int LIM=5e5+7;
ll T[LIM];
int main() {
	ios_base::sync_with_stdio(0); cin.tie(0);
	ll n, k, ans=0, akt=0;
	cin >> n >> k;
	rep(i, n) {
		ll a;
		cin >> a;
		++T[a];
		if(T[a]==1) {
			ans+=i-akt;
			++akt;
			if(akt==k) break;
		}
	}
	cout << (akt==k?ans:-1) << '\n';
}