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#include <bits/stdc++.h>
using namespace std;

#define REP(i, n) for (int i = 0; i < (n); ++i)
#define SZ(a) ((int)(a).size())
#define ALL(a) (a).begin(), (a).end()

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;

constexpr int kInf = 1000 * 1000 * 1000 + 7;
constexpr long long kInfLL = 1e18;

// #include <atcoder/fenwicktree>
// using namespace atcoder;
// using mint = modint998244353;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  ll result = 0;

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

  vi a(n);
  REP(i, n) cin >> a[i];

  set<int> s;
  REP(i, n) {
    int x = SZ(s);
    s.insert(a[i]);
    if (x < k && x < SZ(s)) {
      result += i - x;
    }
  }

  if (SZ(s) < k) {
    cout << -1 << endl;
    return 0;
  }

  cout << result << endl;
}