#include <bits/stdc++.h>
template <typename T>
std::istream& operator>>(std::istream& input, std::vector<T>& v) {
  for (T& a : v) input >> a;
  return input;
}
void answer(unsigned v) { std::cout << v << '\n'; }
void solve(unsigned n, unsigned k, const std::vector<unsigned>& a) {
  unsigned t = 0, p = 0;
  for (const unsigned x : a) {
    p += (x - t - 1) / k + 1;
    t = x;
  }
  answer(p + (n - t) / k);
}
int main() {
  unsigned n, k;
  std::cin >> n >> k;
  size_t c;
  std::cin >> c;
  std::vector<unsigned> a(c);
  std::cin >> a;
  solve(n, k, a);
  return 0;
}
