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

int main() {
  int n, k;
  cin >> n >> k;
  set<int> left;
  long long result = 0;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (left.count(x) == 0 && left.size() < k) {
        left.insert(x);
        result = result + i - left.size() + 1;
    }
  }
  if (left.size() == k) {
      cout << result;
  } else {
      cout << -1;
  }
}