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

#define int long long
#define st first
#define nd second
#define bg begin
#define ed end
#define pb push_back
#define all(r) bg(r),ed(r)
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0) 

int32_t main(){
	fast;
	int n,k;
	cin>>n>>k;
	unordered_map<int, bool>mapa;
	vector<int>ind;
	int count=0;
	int res=0;
	for(int i=0;i<n;i++){
		int x;
		cin>>x;
		if(mapa[x]==0){
			mapa[x]=1;
			res+=i-count;
			count++;
		}
		if(count==k){
			cout<<res<<'\n';
			return 0;
		}
	}
	cout<<-1<<'\n';	
}