#include <bits/stdc++.h>
using namespace std;
int main() {
  unsigned int n, k, i, c, num, j;
  string s;
  cin >> n >> k;
  int a[n + 1];
  cin >> c;
  for (i = 1; i <= c; i++) {
    cin >> a[i];
  }
  num = 0;
  j = 1;
  a[0] = 0;
  for (i = 1; i <= c; i++) {
    num += ((a[i] - a[i - 1] - 1) / k) + 1;
  }
  num += (n - a[i - 1]) / k;
  cout << num << endl;
  return 0;
}
