#include <bits/stdc++.h>
using namespace std;
struct iii {
  int a, b, c;
  iii() {}
  iii(int a, int b, int c) : a(a), b(b), c(c) {}
  bool operator<(const iii& e) const {
    return a < e.a || a == e.a && (b < e.b || b == e.b && c < e.c);
  }
};
template <typename T>
T sq(const T& x) {
  return x * x;
}
template <typename T>
void minl(T& a, const T& b) {
  if (a > b) a = b;
}
template <typename T>
void maxl(T& a, const T& b) {
  if (a < b) a = b;
}
template <typename T>
void operator+=(vector<T>& a, const T& x) {
  a.push_back(x);
}
template <typename T>
void operator+=(set<T>& a, const T& x) {
  a.insert(x);
}
template <typename T>
bool operator&(const set<T>& a, const T& x) {
  return a.find(x) != a.end();
}
template <typename A, typename B>
istream& operator>>(istream& i, pair<A, B>& v) {
  return i >> v.first >> v.second;
}
pair<int, int> operator+(const pair<int, int>& a, const pair<int, int>& b) {
  return pair<int, int>(a.first + b.first, a.second + b.second);
}
pair<int, int> operator-(const pair<int, int>& a, const pair<int, int>& b) {
  return pair<int, int>(a.first - b.first, a.second - b.second);
}
template <typename T>
void usort(vector<T>& a) {
  sort((a).begin(), (a).end());
  a.erase(unique((a).begin(), (a).end()), a.end());
}
template <typename T>
int index(const vector<T>& a, const T& x) {
  return lower_bound((a).begin(), (a).end(), x) - a.begin();
}
template <typename T>
T gcd(T a, T b) {
  while (b) {
    T t = a % b;
    a = b;
    b = t;
  }
  return a;
}
long long cross(const pair<int, int>& a, const pair<int, int>& b) {
  return (long long)a.first * b.second - (long long)b.first * a.second;
}
long long dot(const pair<int, int>& a, const pair<int, int>& b) {
  return (long long)a.first * b.first + (long long)a.second * b.second;
}
int bitcnt(unsigned x) { return __builtin_popcount(x); }
int bitlow(unsigned x) { return __builtin_ffs(x); }
void york() {
  int n, k, c;
  vector<int> a;
  cin >> n >> k >> c;
  for (int i = 0; i < (c); ++i) {
    int t;
    cin >> t;
    a += t;
  }
  int ans = c;
  if (c == 0) {
    ans = n / k;
  } else {
    for (int i = 1; i < c; i++) {
      ans += (a[i] - a[i - 1] - 1) / k;
    }
    ans += (a[0] - 1) / k;
    ans += max(n - a[c - 1], 0) / k;
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  york();
  return 0;
}
