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#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

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

  int n, k, b, i, j;
  long result = 0;
  cin >> n >> k;
  auto visited = vector<bool>(n + 1, false);
  cin >> b;
  visited[b] = true;
  for (i = 0, j = 1; j < n && i + 1 < k; j++) {
    cin >> b;
    if (visited[b])
      continue;
    visited[b] = true;
    result += j - i - 1;
    i += 1;
  }
  cout << (i + 1 == k ? result : -1) << endl;
  return 0;
}