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#include <bits/stdc++.h>
using namespace std;

using ll = long long;

int main() {
  int n, k;
  scanf("%d %d", &n, &k);
  vector<int> a(n);
  vector<char> vis(n+1);
  int diff = 0;
  for (auto &i : a) {
    scanf("%d", &i);
  }
  ll res = 0;
  for (int i = 0; i < n && diff < k; i++) {
    if (vis[a[i]]) {
      continue;
    }
    res += i - diff;
    diff++;
    vis[a[i]] = 1;
  }
  printf("%lld", diff == k ? res : -1);
}