#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
template <class T>
bool smin(T &a, T b) {
  return a > b ? (a = b, true) : false;
}
template <class T>
bool smax(T &a, T b) {
  return a < b ? (a = b, true) : false;
}
const int N = 370;
int arr[N];
int main(void) {
  int n, k;
  scanf("%d%d", &n, &k);
  int d;
  scanf("%d", &d);
  for (int i = 0; i < d; i++) scanf("%d", &arr[i]);
  sort(arr, arr + d);
  int prv = 1;
  int ans = d;
  for (int i = 0; i < d; i++) {
    ans += (arr[i] - prv) / k;
    prv = arr[i] + 1;
  }
  ans += (n - arr[d - 1]) / k;
  printf("%d\n", ans);
  return 0;
}
