#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:836777216")
using namespace std;
int n, k, c;
bool u[1111];
int main() {
  cin >> n >> k;
  cin >> c;
  for (int i(0); i < (c); i++) {
    int x;
    cin >> x;
    u[x] = 1;
  }
  int num = 0;
  int end = k;
  for (int i(1); i < (n + 1); i++) {
    if (u[i])
      num++, end = i + k;
    else {
      if (i == end) num++, end = i + k;
    }
  }
  cout << num << endl;
  return 0;
}
