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#include <bits/stdc++.h>
#define int long long
#define mp make_pair
#define pb push_back
#define ld long double
#define pii pair<int,int>
#define sz(x) (int)x.size()
#define piii pair<pii,pii>
#define precise cout<<fixed<<setprecision(10)
#define st first
#define nd second
#define ins insert
#define vi vector<int>
#define BOOST ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
using namespace std;
const int MAX=5e5+5;
const int inf=1e18+9;
int a[MAX];
int p[MAX];
int32_t main(){
  BOOST;
  int n,k;
  cin>>n>>k;
  for (int i=1;i<=n;i++)p[i]=inf;
  for (int i=1;i<=n;i++)cin>>a[i];
  for (int i=n;i>=1;i--)p[a[i]]=i;
  sort(p+1,p+n+1);
  int ans=0;
  for (int i=1;i<=k;i++){
    if (p[i]==inf){
			cout<<"-1";
			return 0;
    }
		ans+=p[i]-i;
	}
  cout<<ans;
  return 0; 
}