#include <bits/stdc++.h>
using namespace std;
int flag[400];
int main() {
  int n, k, i, c, sum = 0, x;
  cin >> n >> k >> c;
  for (i = 1; i <= c; i++) {
    cin >> x;
    flag[x] = 1;
  }
  int j = 0;
  for (i = 1; i <= n; i++) {
    j++;
    if (flag[i] == 1 || j == k) {
      j = 0;
      sum++;
    }
  }
  cout << sum << endl;
  return 0;
}
