#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, c, x = 0, x2 = 0, res = 0;
  scanf("%d%d%d", &n, &k, &c);
  for (int i = 0; i < c; i++) {
    scanf("%d", &x);
    res += (x - x2 - 1) / k;
    x2 = x;
  }
  res += (n - x) / k;
  printf("%d", res + c);
}
