#include <bits/stdc++.h>
using namespace std;
long long d[510];
int main() {
  long long n, k;
  cin >> n >> k;
  long long c;
  cin >> c;
  for (long long i = 0; i < c; i++) {
    long long x;
    cin >> x;
    d[x] = 1;
  }
  long long pos = 0;
  while (pos <= n) {
    long long i;
    for (i = pos + 1; i <= pos + k; i++) {
      if (d[i]) {
        pos = i;
        break;
      }
    }
    if (i > pos + k) {
      pos = pos + k;
      d[pos] = 1;
    }
  }
  long long ans = 0;
  for (long long i = 1; i <= n; i++) ans += d[i];
  cout << ans << '\n';
  return 0;
}
