#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  ios_base::sync_with_stdio(0);
  cin.tie(0), cout.tie(0);
  int n, k, c, ans = 0, x;
  cin >> n >> k >> c;
  vector<bool> v(n + 1, 0);
  while (c--) cin >> x, v[x] = 1;
  int cnt = 0;
  for (int i = 0; i <= n; i++) {
    if (v[i] == true)
      ans++, cnt = 0;
    else if (cnt == k)
      v[i] = 1, cnt = 0, ans++;
    cnt++;
  }
  cout << ans;
}
