#include <bits/stdc++.h>
const int N = 365;
int n, k, m;
int a[N], b, c, d;
int kol;
int main() {
  int i, j, bb;
  scanf("%d %d %d", &n, &k, &c);
  for (i = 0, b = 0; i < c; i++) {
    scanf("%d", &a[i]);
    bb = b;
    if (a[i] < b + k)
      kol++;
    else
      kol += (a[i] - b + k - 1) / k;
    b = a[i];
  }
  kol += (n - b) / k;
  printf("%d", kol);
  return 0;
}
