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#include <bits/stdc++.h>
#include <random>
#define ll long long int
#define pb push_back
#define st first
#define nd second
#define pii pair<int,int>
#define mp make_pair
#define pll pair<long long,long long>
#define ld long double
#define ull unsigned long long

using namespace std;

const int nax = 5e5 + 5;
int a[nax];
int n, k;
int cnt[nax];
int met[nax];

void solve(){
    cin >> n >> k;
    for(int i=1;i<=n;i++) cin >> a[i];
    for(int i=1;i<=n;i++){
        cnt[a[i]] += 1;
    }
    int u = 0;
    for(int i=0;i<nax;i++){
        u += (cnt[i] > 0);
    }
    if(u < k){
        cout << -1 << "\n";
        return;
    }
    ll ans = 0;
    int done = 0;
    for(int i=1;i<=n;i++){
        if(done == k) break;
        int col = a[i];
        if(met[col] > 0) continue;
        met[col] = 1;
        ans += (i - (done + 1));
        done += 1;
    }
    cout << ans << "\n";

}

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0);

    int tt = 1;
    // cin >> tt;
    while(tt--) solve();

    return 0;
}