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#include <bits/stdc++.h>
using namespace std;
#define nd second
#define st first
#define pii pair<int,int>
#define pb push_back
typedef long long ll;


int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(); cout.tie();
    
    int n, k;
    cin>>n>>k;

    vector<int>v(n);
    for(int i=0; i<n; i++)
        cin>>v[i];

    set<int>s;
    ll cnt=0, ans=0;
    for(int i=0; i<n; i++){
        if(s.find(v[i])!=s.end()) continue;
        if(cnt==k) break;

        ans+=(i-cnt);
        cnt++;
        s.insert(v[i]);
    }

    if(cnt<k) cout<<"-1";
    else cout<<ans;
}