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// Daniel Grzegorzewski
// while (clock()<=69*CLOCKS_PER_SEC)
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
// #pragma GCC optimize("Ofast")
// #pragma GCC target("avx,avx2,fma")

#define MP make_pair
#define PB push_back
#define ST first
#define ND second
#define int long long

using namespace __gnu_pbds;
using namespace std;

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

//X.find_by_order(k); - zwraca iterator na k-ty element (numeracja od zerowego)
//X.order_of_key(k); - zwraca liczbę elementów ostro mniejszych niż k

typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<PII> VII;
typedef long long LL;

void init_ios() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}

auto random_address = [] { char *p = new char; delete p; return uint64_t(p); };
 
const uint64_t SEED = chrono::steady_clock::now().time_since_epoch().count() * (random_address() | 1);
mt19937_64 rng(SEED);

const int N = 5*(int)1e5 + 3;

int n, k, nxt, res, a[N];
bool vis[N];

signed main() {
  init_ios();
  cin >> n >> k;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    if (k > 0 && !vis[a[i]]) {
      res += i-nxt;
      --k;
      ++nxt;
    }
    vis[a[i]] = true;
  }
  if (k)
    res = -1;
  cout<<res<<"\n";
}