#include <bits/stdc++.h>
using namespace std;
template <typename T>
void maxtt(T& t1, T t2) {
  t1 = max(t1, t2);
}
template <typename T>
void mintt(T& t1, T t2) {
  t1 = min(t1, t2);
}
bool debug = 0;
int n, m, k;
int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
string direc = "URDL";
long long ln, lk, lm;
void etp(bool f = 0) {
  puts(f ? "YES" : "NO");
  exit(0);
}
void addmod(int& x, int y, int mod = 1000000007) {
  assert(y >= 0);
  x += y;
  if (x >= mod) x -= mod;
  assert(x >= 0 && x < mod);
}
void et(int x = -1) {
  printf("%d\n", x);
  exit(0);
}
long long fastPow(long long x, long long y, int mod = 1000000007) {
  long long ans = 1;
  while (y > 0) {
    if (y & 1) ans = (x * ans) % mod;
    x = x * x % mod;
    y >>= 1;
  }
  return ans;
}
long long gcd1(long long x, long long y) { return y ? gcd1(y, x % y) : x; }
int dp[1000035][3][3];
void fmain(int tid) {
  scanf("%d%d", &n, &m);
  vector<int> b(m + 1, 0);
  for (int(i) = 1; (i) <= (int)(n); (i)++) {
    scanf("%d", &k);
    b[k]++;
  }
  memset(dp, -1, sizeof dp);
  dp[0][0][0] = 0;
  for (int(i) = 1; (i) <= (int)(m); (i)++) {
    for (int(j) = 0; (j) < (int)(3); (j)++)
      for (int(k) = 0; (k) < (int)(3); (k)++)
        if (dp[i - 1][j][k] > -1) {
          for (int(t3) = 0; (t3) < (int)(3); (t3)++)
            if (j + k + t3 <= b[i]) {
              maxtt(dp[i][k][t3],
                    dp[i - 1][j][k] + (b[i] - j - k - t3) / 3 + t3);
            }
        }
  }
  int ans = dp[m][0][0];
  printf("%d\n", ans);
}
int main() {
  int t = 1;
  for (int(i) = 1; (i) <= (int)(t); (i)++) {
    fmain(i);
  }
  return 0;
}
