#include <bits/stdc++.h>
using namespace std;
int n, k, c;
int d[400];
int main() {
  while (scanf("%d%d", &n, &k) == 2) {
    scanf("%d", &c);
    int ans = c;
    for (int i = 0; i < c; ++i) {
      scanf("%d", &d[i]);
      if (i == 0 && d[0] > k)
        ans += (d[0] - 1) / k;
      else if (i != 0 && d[i] - d[i - 1] > k)
        ans += (d[i] - d[i - 1] - 1) / k;
    }
    if (n - d[c - 1] >= k) ans += (n - d[c - 1]) / k;
    printf("%d\n", ans);
  }
}
