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

const LL N = 5e5 + 10;

bool met[N + 1];

int main() {
  LL n, k, at = 0, res = 0; cin >> n >> k;
  for (LL i = 0; i < n; ++i) {
    LL x; cin >> x;
    if (at < k && !met[x]) {
      res += i - at++;
      met[x] = true;
    }
  }
  cout << (at < k ? -1 : res) << '\n';
  return 0;
}