#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
const int INF = (int)1e9;
const long long LINF = (long long)1e18;
const long double PI = 2 * acos((long double)0);
long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long fpow(long long n, long long k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
template <class T>
void setmin(T& a, T val) {
  if (a > val) a = val;
}
template <class T>
void setmax(T& a, T val) {
  if (a < val) a = val;
}
void addmod(int& a, int val, int p = MOD) {
  if ((a = (a + val)) >= p) a -= p;
}
void submod(int& a, int val, int p = MOD) {
  if ((a = (a - val)) < 0) a += p;
}
int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }
int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }
const int maxn = 100010;
int n, m;
int a[maxn];
int b[maxn];
int l[maxn];
int r[maxn];
int f[maxn];
int g[maxn];
void solve() {
  scanf("%d%d", &n, &m);
  for (int i = (1); i < (n + 1); i++) {
    scanf("%d", a + i);
    l[i] = r[i] = i;
  }
  for (int i = (0); i < (m); i++) scanf("%d", b + i);
  sort(a + 1, a + n + 1);
  sort(b, b + m);
  for (int i = (2); i < (n + 1); i++) {
    if (a[i] == a[i - 1] + 1) {
      l[i] = l[i - 1];
    }
  }
  for (int i = (1); i < (n + 1); i++) {
    int k = upper_bound(b, b + m, a[i]) - b;
    for (int j = (0); j < (k); j++)
      if (i - a[i] + b[j] >= 0 && l[i - a[i] + b[j]]) {
        setmax(g[i], f[l[i - a[i] + b[j]] - 1] + k - j);
      }
    setmax(f[i], f[i - 1]);
    setmax(f[i], g[i]);
    setmax(g[i], g[i - 1]);
    if (l[i] == i) setmax(g[i], f[i - 1]);
    for (int j = (k); j < (m); j++) {
      if (i + b[j] - a[i] <= n) {
        setmax(f[i + b[j] - a[i]], g[i] + j - k + 1);
      }
    }
  }
  printf("%d", f[n]);
}
int main() {
  solve();
  return 0;
}
