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// Grzegorz Ryn
// V LO Kraków

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;

#define pb push_back
#define eb emplace_back
#define mp make_pair
#define mt make_tuple
#define st first
#define nd second
#define FOR(__VAR, __START, __END) for(int __VAR=__START; __VAR<=__END; __VAR++)
#define FORB(__VAR, __START, __END) for(int __VAR=__START; __VAR>=__END; __VAR--)
#define FORK(__VAR, __START, __END, __DIFF) for(int __VAR=__START; __VAR<=END; __VAR+=__DIFF)
#define all(__VAR) (__VAR).begin(), (__VAR).end()
#define rall(__VAR) (__VAR).rbegin(), (__VAR).rend()
#define DEBUG(__VAR) cout << #__VAR << ": " << __VAR << endl;

template<typename __T1, typename __T2>
ostream & operator<<(ostream &out, pair<__T1, __T2> &__VAR) {
	cout << "[" << __VAR.st << ", " << __VAR.nd << "]";
	return out;
}

template<typename __T>
ostream & operator<<(ostream &out, vector<__T> &__VAR) {
	cout << "[";
	FOR(i, 0, (int)__VAR.size()-2)
		cout << __VAR[i] << ", ";
	if(__VAR.size()>0)
		cout << __VAR[__VAR.size()-1];
	cout << "]" << endl;
	
	return out;
}

const int INF=1e9;

ll solve() {
	int N, K;
	cin >> N >> K;
	vector<int> a(N);
	for(int i=0; i<N; i++) {
		cin >> a[i];
	}
	
	ll result = 0;
	int cnt = 0;
	vector<bool> vis(N+1, false);
	for(int i=0; i<N && cnt < K; i++) {
		if(vis[a[i]]) {
			continue;
		}
		vis[a[i]] = true;
		result += (ll)(i-cnt);
		cnt++;
	}
	
	if(cnt < K) {
		return -1;
	}
	return result;
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	cout << solve() << '\n';

	return 0;
}