#include <bits/stdc++.h>
using namespace std;
const int inf = (1 << 30);
int n, k, c, i, j, x, f, ans;
bool a[1000];
int main() {
  cin >> n >> k;
  cin >> c;
  for (i = 0; i < c; i++) {
    cin >> x;
    a[x] = 1;
  }
  a[0] = 1;
  for (i = 0; i <= n; i++) {
    if (a[i]) {
      ans++;
      for (j = i + 1; j <= i + k; j++)
        if (a[j]) goto br;
      a[i + k] = 1;
    }
  br:;
  }
  cout << ans - 1;
  return 0;
}
