#include <bits/stdc++.h>
long long MOD = 1000000007;
long long inf = 1e18;
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  int n, k, c, i, p, x, ans = 0;
  cin >> n >> k >> c;
  p = 0;
  for (i = 0; i < c; i++) {
    cin >> x;
    while (x - p > k) {
      p += k;
      ans++;
    }
    ans++;
    p = x;
  }
  while (n - p + 1 > k) {
    p += k;
    ans++;
  }
  cout << ans;
  return 0;
}
