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#include <bits/stdc++.h>
#define boost ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define debug(x) cerr << #x << " " << x << endl
#define int long long
#define st first
#define nd second

using namespace std;

const int N(5e5 + 13);
const int MAX(LLONG_MAX);

int n, k, current_position = 1, count_bottles, res;
int arr[N], vis[N];
bool flag;
string s;

int32_t main() {
  boost;
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> arr[i];
    if(!vis[arr[i]] && count_bottles < k) {
      res += i - current_position;
      current_position++;
      vis[arr[i]] = 1;
      count_bottles++;
    }
  }

  if(count_bottles == k) {
    cout << res;
  } else {
    cout << "-1";
  }

}