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

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

  int n, k; cin >> n >> k;
  int count_unique = 0;
  long long res = 0;
  vector<bool> bset(n+1);
  int leftmost_free = 0;
  vector<int> free_positions;

  for (int i = 0; i < k; ++i) {
    int a; cin >> a;
    if (bset[a]) 
      free_positions.push_back(i);
    else bset[a] = true;
  }

  const int sz = free_positions.size();
  auto prefix_free = [&leftmost_free, &sz](){ return leftmost_free < sz; };
  for (int i = k; i < n and prefix_free(); ++i) {
    int a; cin >> a;
    if (not bset[a]) {
      res += i - free_positions[leftmost_free++];
      bset[a] = true;
    }
  }

  if (prefix_free())
    cout << -1;
  else
    cout << res;
}