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#include <bits/stdc++.h> 

typedef long long int lli;

using namespace std;

#define MAX4 10010
#define MAX5 100010
#define MAX6 1000010

template <typename T >
void dbg(T last)
{
  cout << last << "\n";
}

template <typename T, typename ...args>
void dbg(T current, args ...next)
{
  cout << current << ' ';
  dbg(next...);
}

//-----------------------------------------------------------------------------------------------//
int tab[MAX6];
bool vis[MAX6];

void solve()
{
  int n, k;

  cin >> n >> k;

  int d = 0, dk =0;

  for (int i=0;i<n;i++)
  {
    cin >> tab[i];

    if (!vis[tab[i]])
    {
      d++;

      if (i < k)
      {
        dk++;
      }

      vis[tab[i]] = true;
    }
  }

  if (d < k)
  {
    cout << "-1";
    return;
  }
  else if (dk == k)
  {
    cout << "0";
    return;
  }

  lli sum = 0;

  for (int i=0;i<n;i++)
  {
    vis[tab[i]] = false;
  }

  int last;

  for (int i=0;i<k;i++)
  {
    if (vis[tab[i]])
    {
      last = i;

      break;
    }

    vis[tab[i]] = true;
  }

  for (int i=last+1;i<n;i++)
  {
    if (!vis[tab[i]] && last < k)
    {
      sum += (lli)i - (lli)last++;
    }

    vis[tab[i]] = true;
  }

  cout << sum;
}

int main()
{
  std::ios::sync_with_stdio(false);

  solve();

  return 0;
}