#include <bits/stdc++.h>
using namespace std;
int n, i, j, k, c[366], c1, cnt = 0, l = 0;
int main() {
  scanf("%d%d%d", &n, &k, &c1);
  for (i = 0; i < c1; i++) {
    scanf("%d", &c[i]);
  }
  for (i = 1; i <= n; i++) {
    l++;
    if (l == k) {
      cnt++;
      l = 0;
      continue;
    }
    for (j = 0; j < c1; j++) {
      if (i == c[j]) {
        cnt++;
        l = 0;
        break;
      }
    }
  }
  printf("%d\n", cnt);
  return 0;
}
