// Patryk Jędrzejczak

#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for(int i = 0; i < n; ++i)
#define REPR(i, n) for(int i = n - 1; i >= 0; --i)
#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define FORR(i, a, b) for(int i = b - 1; i >= a; --i)
#define EB emplace_back
#define MP make_pair
#define ST first
#define ND second
#define S size
#define RS resize
#define endl '\n'
#define L long long

template<class T> using V = vector<T>;

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

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

  vector<int> a(n);
  vector<int> first(n, n);
  int different = 0;
  REP(i, n) {
    cin >> a[i];
    a[i]--;
    if (first[a[i]] == n) {
      different++;
      first[a[i]] = i;
    }
  }

  if (different < k) {
    cout << -1;
    return 0;
  }

  vector<bool> is_first(n);
  REP(i, n) {
    if (first[i] != n) {
      is_first[first[i]] = true;
    }
  }

  L result = 0;
  int found = 0;
  int position = 0;
  while (found < k) {
    if (is_first[position]) {
      result += (L)(position - found);
      found++;
    }
    position++;
  }
  cout << result;
}