#include <bits/stdc++.h>
using namespace std;
const int MAXN = 365 + 5;
bool mark[MAXN];
int n, k, c;
int main() {
  cin >> n >> k >> c;
  for (int i = 1; i <= c; i++) {
    int tmp;
    cin >> tmp;
    mark[tmp] = true;
  }
  int res = 0, rem = 0;
  for (int i = 1; i <= n; i++) {
    rem++;
    if (rem == k || mark[i]) {
      rem = 0;
      res++;
    }
  }
  cout << res << endl;
  return 0;
}
