#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c;
  int h[400];
  cin >> n >> k >> c;
  for (int(i) = 0; (i) < (c); ++(i)) {
    cin >> h[i];
  }
  int d = k;
  int ans = 0;
  for (int(i) = (1); (i) <= (n); ++(i)) {
    if (i == d) {
      ans++;
      d = i + k;
    } else {
      int flag = false;
      for (int(j) = 0; (j) < (c); ++(j)) {
        if (h[j] == i) {
          flag = true;
          break;
        }
      }
      if (flag) {
        ans++;
        d = i + k;
      }
    }
  }
  cout << ans;
  return 0;
}
