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

const int k = 5*1e5 + 5;

pair <long long int,long long int> tab[k];
bool vis[k];

int main(){
	
	long long int n, m, ans = 0, poz = 1;
	cin>>n>>m;
	
	for(int i=0; i<n; i++){
		
		int a;
		cin>>a;
		
		tab[i] = { i , a };
		
	} 
	
	sort(tab, tab+n);
	
	vis[tab[0].second] = true;
	
	for(int i=1; i<n; i++){
		
		if(poz == m) break;
		
		if(vis[tab[i].second])continue;
		else{
			
			//cout<<tab[i].second<<' '<<poz<<" "<<tab[i].first<<"\n";
			
			ans += tab[i].first - poz;
			vis[tab[i].second] = true;
			poz++;
			
		}
		
	}
	
	if(poz == m) cout<<ans;
	else cout<<"-1";
	
}