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#include <stdio.h>
#include <vector>
#include <map>
#include <unordered_set>
#include <queue>
#include <set>
#include <unordered_map>
#include <math.h>
#include <limits.h>
#include <algorithm>
#include <functional>
#include <iterator>
#include <algorithm>
#include <string>
#include <iostream>

using namespace std;

#define pb push_back
#define mp make_pair

typedef long long ll;
typedef unsigned long long ull;

using namespace std;

//ora.in3 -> 2137

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

  int n, k, ai;
  cin >> n >> k;

  set<int> diff;
  vector<int> bts;
  for (int i=0; i<n; ++i) {
    cin >> ai;
    bts.pb(ai);
    diff.insert(ai);
  }

  if (diff.size() < k) {
    cout << -1 << endl;
  } else {
    ll ops = 0;

    set<int> dups;
    dups.insert(bts[0]);
    k--;
    int curr = 1, unique = 1;

    while (k > 0) {
      while (dups.find(bts[unique]) != dups.end()) unique++;
      ops += (unique - curr);
      dups.insert(bts[unique]);
      curr++;
      unique++;
      k--;
    }

    cout << ops << endl;
  }

  return 0;
}