#include <bits/stdc++.h>
using namespace std;
bool d[400];
void input() {
  int i, n, nk, nc, x;
  scanf("%d %d", &n, &nk);
  scanf("%d", &nc);
  for ((i) = 0; (i) < (nc); (i)++) scanf("%d", &x), d[x] = true;
  int res = 0, cnt = 0;
  for ((i) = (1); (i) <= (n); (i)++) {
    cnt++;
    if (cnt == nk || d[i]) {
      res++;
      cnt = 0;
    }
  }
  printf("%d", res);
}
int main() {
  input();
  return 0;
}
