#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, ans;
  int d, r;
  bool hld[400], f;
  memset(hld, 0, sizeof(hld));
  scanf("%d%d", &n, &k);
  scanf("%d", &c);
  for (int i = 0; i < c; ++i) {
    scanf("%d", &d);
    hld[d] = true;
  }
  ans = c;
  d = 0;
  while (d < n) {
    r = k;
    f = false;
    while (r > 0 && d < n) {
      ++d;
      --r;
      if (hld[d] == true) {
        f = true;
        break;
      }
    }
    if (!f && r == 0) ++ans;
  }
  printf("%d\n", ans);
}
