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#include <algorithm>
#include <iostream>
using namespace std;
const int MAXN = 500000;

#define FOR(i, n) for(int i = 0, __n = (n); i < __n; i++)



int p[MAXN];
bool f[MAXN];


int main() {
  ios_base::sync_with_stdio(0);

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

  FOR(i, n) {
    cin >> p[i];
    p[i]--;
    f[i] = false;
  }

  int s = 0;
  long long res = 0;

  FOR(i, n) {
    if(s == k) break;
    if (!f[p[i]]) {
      res += i-s;
      f[p[i]] = true;
      s++;
    }
  }
  
  cout << (s<k  ? -1 : res) << endl;;
}