#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, k;
  scanf("%d%d", &n, &k);
  scanf("%d", &m);
  int ans = m;
  int xp = 0;
  for (int i = 0; i < m; i++) {
    int x;
    scanf("%d", &x);
    xp = (x - xp - 1) / k;
    ans += xp;
    xp = x;
  }
  xp = (n - xp) / k;
  ans += xp;
  printf("%d\n", ans);
  return 0;
}
