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#include <bits/stdc++.h>
#include<algorithm>
#include <math.h> 
using namespace std;
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define pb push_back
#define pii pair<int, pair<int, int>>
#define mp make_pair
#define f first
#define s second
#define all(x) (x).begin(), (x).end()

int main() {
  fast_io;
  int n, k;
  vector<int> l;
  cin >> n >> k;
  int last;
  int cnt = 0;
  vector<bool> include(n, false);
  for (int i = 0; i < n; i++){
    int x;
    cin >> x;
    l.pb(x);
  } 
  vector<bool> seen(n, false);
  for (int i = 0; i < n; i++){
    if (seen[l[i]] == false) {
      include[i] = true;
      if (cnt == k - 1) {
        last = i;
        cnt += 1;
        break;
      }
      else {
        cnt += 1;
      }
      seen[l[i]] = true;
    }
  }
  if (cnt < k) {
    cout << -1;
  }
  else {
    long long ans = 0;
    int sub = 0;
    for (int i = last - 1; i > 0; i--) {
      if (!include[i]) {
        ans += (last - i - sub);
        sub += 1;
      }
    }
    cout << ans;
  }
}