#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, start, end, i, ans;
  cin >> n >> k >> c;
  start = 0;
  ans = c;
  while (c--) {
    cin >> end;
    ans += (end - start - 1) / k;
    start = end;
  }
  ans += (n - start) / k;
  cout << ans << endl;
  return 0;
}
