#include <bits/stdc++.h>
using namespace std;
int arr[366];
int main() {
  int N;
  int K;
  scanf("%d %d", &N, &K);
  int C;
  scanf("%d", &C);
  for (int n = 0; n < C; n++) {
    scanf("%d", &arr[n]);
  }
  int pos = 1;
  int counter = 0;
  int p = 0;
  int res = 0;
  while (pos <= N) {
    counter++;
    if (p < C && arr[p] == pos) {
      p++;
      res++;
      counter = 0;
    } else if (counter == K) {
      counter = 0;
      res++;
    }
    pos++;
  }
  printf("%d\n", res);
}
