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#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define pb push_back
#define st first
#define nd second

bool vis[500009];
vector<int> v;
int tab[500009];

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	int n, k;
	cin >> n >> k;
	for(int i=0;i<n;i++)
		cin >> tab[i];
	for(int i=0;i<k;i++) {
		if(vis[tab[i]])
			v.pb(i);
		vis[tab[i]] = true;
	}
	ll r = 0;
	for(int i=k;i<n;i++) {
		if(v.size() and !vis[tab[i]]) {
			vis[tab[i]] = true;
			r+=i-v.back();
			v.pop_back();
		}
	}
	if(v.size()) {
		cout << "-1\n";
		return 0;
	}
	cout << r << "\n";
}