#include <bits/stdc++.h>
using namespace std;
const int N = 1e2 + 2;
const int INF = 1e9 + 7;
const int MOD = 1e9 + 7;
const double EPS = 1e-8;
int main() {
  int n, k;
  scanf("%d", &n);
  scanf("%d", &k);
  int c;
  scanf("%d", &c);
  int lp = 0;
  int np = 0;
  for (int i = 0; i < c; i++) {
    int v;
    scanf("%d", &v);
    while (lp + k < v) {
      lp += k;
      np++;
    }
    lp = v;
    np++;
  }
  while (lp + k <= n) {
    lp += k;
    np++;
  }
  cout << np << "\n";
}
