#include <bits/stdc++.h>
bool used_[366];
int main() {
  int n, k, c;
  std::scanf("%d%d%d", &n, &k, &c);
  while (c--) {
    int a;
    std::scanf("%d", &a);
    used_[a] = true;
  }
  int count = 0;
  for (int i = 1, pi = 0; i <= n; ++i) {
    if (!used_[i] && i - pi < k) continue;
    ++count;
    pi = i;
  }
  std::printf("%d\n", count);
  return 0;
}
