#include <bits/stdc++.h>
using namespace std;
int main() {
  int j[365];
  int days, i, k, h, d, c;
  d = 0;
  c = 0;
  cin >> days;
  cin >> k;
  cin >> h;
  for (i = 0; i < h; i++) cin >> j[i];
  i = 0;
  while (d + k <= days || i < h) {
    if (i == h || d + k < j[i]) {
      d += k;
      c++;
    } else {
      d = j[i];
      c++;
      i++;
    }
  }
  cout << c;
  return 0;
}
