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#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define SIZE(x) (int)x.size()
#define REP(x,t) for(ll x=0; x<(t); x++)
#define FOR(x,t) for(ll x=1; x<=(t); x++)
#define pb(x) push_back(x)
#define pf(x) push_front(x)
#define st first
#define nd second
ll inf=1e9+7;
ll mod=1e9+7;
int tab[500005];
int main ()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int n,k;
	ll w=0;
	cin>>n>>k;
	int j=0;
	int x;
	REP(i,n){
		cin>>x;
		if(tab[x]==0){
			if(k>0){
				w+=i-j;
				j++;
				k--;
			}
			tab[x]++;
		}
	}
	if(k>0) cout<<"-1";
	else cout<<w;
		
		
	
	


	return 0;
}