#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, g{}, i, j;
  cin >> n >> k;
  int c;
  cin >> c;
  vector<int> h(c);
  for (auto& i : h) cin >> i;
  for (i = 0, j = -1;; i + k < h[j + 1] ? i = i + k : i = h[++j]) {
    ++g;
    if (j == c - 1) break;
  }
  for (i += k; i <= n; i += k) ++g;
  cout << g - 1 << endl;
}
