#include <bits/stdc++.h>
using namespace std;
int n, m, i, a[999], k, c, t, ans;
int main() {
  cin >> n >> k >> c;
  for (i = 0; i < c; i++) cin >> a[i];
  a[c] = n;
  t = k;
  for (i = 0; i < c; i++) {
    while (t < a[i]) {
      t += k;
      ans++;
    }
    t = a[i] + k;
  }
  while (t <= n) {
    t += k;
    ans++;
  }
  cout << c + ans;
}
