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#include <iostream>
  #include <stdio.h>
  using namespace std;


  const int N = 500000;
  int histogram[N + 1];

  int main() {
    for (int i = 1; i <= N; i++) {
      histogram[i] = 0;
    }

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

    int currentPosition = 1;
    int target = k + 1;
    long long switches = 0;
    int found = 0;

    for (int i = 0; i < n; i++) {
      int x;
      cin >> x;
      if (histogram[x] != 0) {
        if (target > n) {
          printf("-1");
          return 0;
        }
        switches += target - currentPosition;
        target++;
      } else {
        found++;
        if (found == k) {
          printf("%lld", switches);
          return 0;
        }
      }
      histogram[x]++;
      currentPosition++;
    }

    return 0;
  }