#include <bits/stdc++.h>
using namespace std;
int k, n, c;
int v[1111];
int main(void) {
  scanf("%d %d", &n, &k);
  scanf("%d", &c);
  for (int i = 0; i < c; i++) {
    scanf("%d", &v[i]);
  }
  int r = 0;
  int p = 0;
  int d = 0;
  while (d < n) {
    if (p >= c) {
      d += k;
      if (d > n) {
        break;
      }
      r++;
      continue;
    }
    if (d + k < v[p]) {
      d += k;
      r++;
      continue;
    }
    d = v[p];
    p++;
    r++;
  }
  printf("%d\n", r);
  return 48 - 48;
}
