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#include <bits/stdc++.h>

using namespace std;

int n, k;

int main() {
  cin >> n >> k;

  vector<int> v;
  for (int i = 0; i < n; ++i) {
    int a;
    cin >> a;
    v.push_back(a);
  }

  set<int> s, firstK;
  vector<int> stack;
  for (int i = 0; i < k; ++i) {
    if (s.count(v[i])) {
      stack.push_back(i);
    }
    s.insert(v[i]);
    firstK.insert(v[i]);
  }
  reverse(stack.begin(), stack.end());
  int res = 0;
  int toAdd = k - (int)firstK.size();
  // for (int i = 0; i < stack.size(); ++i) {
  //   stack.pop_back();
  // }
  for (int i = k; i < n; ++i) {
    if (toAdd <= 0) {
      break;
    }
    if (!firstK.count(v[i])) {
      // cout << i << " " << stack.back() << endl;
      res += i - stack.back();
      stack.pop_back();
      toAdd--;
    }
    firstK.insert(v[i]);
  }

  cout << (toAdd > 0 ? -1 : res) << endl;
}