#include <bits/stdc++.h>
using namespace std;
const double PI = 3.14159265358979;
const double PI2 = 6.28318530717958;
const double PId2 = 1.570796326794895;
template <typename T>
T sqr(T x) {
  return x * x;
}
template <typename T>
T gcd(T a, T b) {
  return (b ? gcd(b, a % b) : a);
}
inline long long pw(long long n, int p) {
  long long ans = 1;
  for (int i = 0; i < p; i++) ans *= n;
  return ans;
}
inline vector<int> ReadVI(int count) {
  vector<int> arrayname(count);
  for (int i = 0; i < count; i++) cin >> arrayname[i];
  return arrayname;
}
inline vector<long long> ReadVlong(int count) {
  vector<long long> arrayname(count);
  for (int i = 0; i < count; i++) cin >> arrayname[i];
  return arrayname;
}
const int MOD = 100000000;
const int MAXVALUE = 101;
int main() {
  int n, k, c, t;
  cin >> n >> k;
  vector<int> a(n + 1, 0);
  cin >> c;
  for (int i = 0; i < c; i++) {
    cin >> t;
    a[t] = 1;
  }
  c = 0;
  for (int i = 1; i <= n; i++) {
    c++;
    if (a[i])
      c = 0;
    else if (c == k)
      a[i] = 1, c = 0;
  }
  c = 0;
  for (int i = 1; i <= n; i++)
    if (a[i]) c++;
  cout << c;
  return 0;
}
