#include <bits/stdc++.h>
using namespace std;
string tostring(long long x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
int main() {
  int n, k, x, p;
  bool pres[500] = {false};
  scanf("%d%d", &n, &k);
  int m;
  scanf("%d", &m);
  for (int(i) = 0; (i) < (m); (i)++) {
    scanf("%d", &p);
    pres[p] = true;
  }
  int it = 0, c = 0, kk = 0;
  for (int(i) = (1); (i) < (n + 1); (i)++) {
    kk++;
    if (pres[i]) {
      it++;
      kk = 0;
    }
    if (kk == k) {
      it++;
      kk = 0;
    }
  }
  printf("%d\n", it);
  return 0;
}
