#include <bits/stdc++.h>
int cc[1000];
int main() {
  int n, k;
  scanf("%d%d", &n, &k);
  int c;
  scanf("%d", &c);
  int last = 0;
  int rez = c;
  for (int i = 0; i < c; i++) {
    int t;
    scanf("%d", &t);
    while (last + k < t) rez++, last += k;
    last = t;
  }
  while (last + k <= n) rez++, last += k;
  printf("%d", rez);
}
