#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c;
  int i, j;
  vector<int> v;
  cin >> n >> k >> c;
  for (i = 0; i < c; i++) {
    cin >> j;
    v.push_back(j);
  }
  int ans = 0;
  i = 0;
  j = k;
  while (1) {
    if (i < v.size() && v[i] <= j) {
      j = v[i] + k;
      i++;
      ans++;
    } else if (j <= n) {
      ans++;
      j += k;
    } else
      break;
  }
  cout << ans << endl;
  return 0;
}
