#include <bits/stdc++.h>
using namespace std;
inline int read() {
  char ch = getchar();
  if (!(~ch)) return 0;
  int f = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  int x = 0;
  while (ch >= '0' && ch <= '9') {
    x = (x << 3) + (x << 1) + (ch ^ 48);
    ch = getchar();
  }
  return x * f;
}
int a[4];
int main() {
  int n = read(), k = read(), c = read(), ans = 0;
  for (register int i = 1; i <= c; ++i) {
    a[i] = read();
    ans += (a[i] - a[i - 1] - 1) / k + 1;
  }
  printf("%d\n", ans + (n - a[c]) / k);
  return 0;
}
