#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c;
  cin >> n >> k >> c;
  if (k == 1) {
    cout << n;
    return 0;
  }
  vector<int> a(c + 1);
  a[0] = 0;
  for (int i = 1; i <= c; ++i) cin >> a[i];
  a.push_back(n);
  int s = 0;
  for (int i = 1; i <= c + 1; ++i) s += (a[i] - a[i - 1] - 1) / k;
  if (((n - a[c]) % k == 0) && (n != a[c])) ++s;
  cout << s + c;
  return 0;
}
