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