#include <bits/stdc++.h>
using namespace std;
struct amulet {};
int main() {
  int n, k, c;
  scanf("%d%d%d", &n, &k, &c);
  vector<bool> arr(n + 1, false);
  for (int i = 0; i < c; i++) {
    int t;
    scanf("%d", &t);
    arr[t] = true;
  }
  int pre = 1, now = 1;
  while (now <= n) {
    if (arr[now])
      pre = 1;
    else {
      if (pre == k) {
        arr[now] = true;
        pre = 1;
      } else
        pre++;
    }
    now++;
  }
  int ans = 0;
  for (int i = 1; i <= n; i++) {
    ans += arr[i] ? 1 : 0;
  }
  printf("%d\n", ans);
  return 0;
}
