#include <bits/stdc++.h>
using namespace std;
int n, k, ans, t;
bool a[500];
int main() {
  cin >> n >> k >> ans;
  for (int i = 1; i <= ans; i++) {
    cin >> t;
    a[t] = 1;
  }
  a[0] = 1;
  for (int i = 0; i <= n; i++) {
    if (!a[i]) a[i] = 1;
    for (int j = 1; j <= k; j++)
      if (a[i + j]) {
        i += j - 1;
        goto endcyc;
      }
    a[i + k] = 1;
    i += k - 1;
    goto endcyc;
  endcyc:;
  }
  ans = -1;
  for (int i = 0; i <= n; i++) ans += a[i];
  cout << ans;
  return 0;
}
