#include <bits/stdc++.h>
using namespace std;
const int INF = int(1e9);
inline void boost() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}
int main() {
  boost();
  int n, k, m;
  cin >> n >> k >> m;
  if (m == 0) {
    cout << n / k;
    exit(0);
  }
  int a[m];
  set<int> x;
  for (int i = 0; i < m; i++) {
    cin >> a[i];
    x.insert(a[i]);
  }
  int sum = x.size();
  sum += (a[0] - 1) / k;
  for (int i = 0; i < m - 1; i++) {
    if (a[i] != a[i + 1]) {
      sum += (a[i + 1] - 1 - a[i]) / k;
    }
  }
  if (a[m - 1] < n) {
    sum += (n - a[m - 1]) / k;
  }
  cout << sum;
  exit(0);
}
