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#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int,null_type,less<int>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
typedef long long ll;
typedef pair<ll,ll> pll;
typedef pair<ll,int> pli;
typedef pair<int,ll> pil;
typedef pair<int,int> pii;
const ll INFLL=1e18+7;
const int INF=1e9+7;
#define pb push_back
const int NAX=5e5+7;
int n,k;
int arr[NAX];
bool picked[NAX];
int main()
{
	ios_base::sync_with_stdio(0);
	cin>>n>>k;
	for(int i=1;i<=n;++i) cin>>arr[i];
	vector<int> to_move;
	int counter=0;
	for(int i=1;i<=n;++i){
		if(!picked[arr[i]]){
			picked[arr[i]]=1;
			++counter;
			to_move.pb(i);
		}
		if(counter>=k) break;
	}
	if(counter<k){
		cout<<"-1\n";
		return 0;
	}
	int curr=1;
	ll ans=0;
	for(int v:to_move){
		ans+=v-curr;
		++curr;
	}
	cout<<ans<<"\n";
}