#include <bits/stdc++.h>
const int maxn = 3e5 + 100;
const int inf = 0x3f3f3f3f;
using namespace std;
template <typename T>
inline void read(T &s) {
  s = 0;
  T w = 1, ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') w = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    s = (s << 1) + (s << 3) + (ch ^ 48);
    ch = getchar();
  }
  s *= w;
}
template <typename T>
inline void write(T s) {
  if (s < 0) putchar('-'), s = -s;
  if (s > 9) write(s / 10);
  putchar(s % 10 + '0');
}
int a[maxn];
struct tree {
  int l, r;
  long long dat, sum, add;
} t[maxn << 2];
void build(int p, int l, int r) {
  t[p].l = l, t[p].r = r;
  if (l == r) {
    t[p].sum = a[l];
    return;
  }
  int mid = (l + r) >> 1;
  build(p << 1, l, mid);
  build(p << 1 | 1, mid + 1, r);
  t[p].dat = t[p << 1].dat + t[p << 1 | 1].dat;
}
void spread(int p) {
  if (t[p].add) {
    t[p << 1].add += t[p].add;
    t[p << 1 | 1].add += t[p].add;
    t[p << 1].dat += t[p].add;
    t[p << 1 | 1].dat += t[p].add;
    t[p].add = 0;
  }
}
void change(int p, int l, int r, int d) {
  if (l <= t[p].l && r >= t[p].r) {
    t[p].sum += (long long)d * (t[p].r - t[p].l + 1);
    t[p].add += d;
    t[p].dat += d;
    return;
  }
  spread(p);
  int mid = (t[p].l + t[p].r) >> 1;
  if (l <= mid) change(p << 1, l, r, d);
  if (r > mid) change(p << 1 | 1, l, r, d);
  t[p].sum = t[p << 1].sum + t[p << 1 | 1].sum;
  t[p].dat = max(t[p << 1].dat, t[p << 1 | 1].dat);
}
long long ask(int p, int l, int r) {
  if (l <= t[p].l && r >= t[p].r) return t[p].dat;
  spread(p);
  int mid = (t[p].l + t[p].r) >> 1;
  long long val = 0;
  if (l <= mid) val = max(val, ask(p << 1, l, r));
  if (r > mid) val = max(val, ask(p << 1 | 1, l, r));
  return val;
}
int n, k;
int main() {
  cin >> n >> k;
  while (k--) {
    int x;
    cin >> x;
    change(1, max(x - k, 1), min(x + k, n), 1);
  }
  ask(1, 1, n);
  cout << endl;
  return 0;
}
