#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 1, 0, -1, 0, 0};
int dy[] = {1, 0, -1, 0, 0, 0};
int dz[] = {0, 0, 0, 0, 1, -1};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, k, c, a;
  cin >> n >> k >> c;
  set<int> st;
  while (c--) cin >> a, st.insert(a);
  a = 0, c = 0;
  for (int i = 1; i <= n; i++)
    if (st.find(i) != st.end())
      c++, a = 0;
    else if (a == k - 1)
      c++, a = 0;
    else
      a++;
  cout << c;
  return 0;
}
