import std.stdio;
import std.conv;
import std.string;
import std.algorithm;
import std.math;
import std.range;

void main()
{
	int[100] a;
	stdin.readln();
	stdin.readln.strip.split.each!(i => a[i.to!int]++);

	int m;
	iota(99).each!(i => m = max(m, a[i] + a[i + 1]));

	writeln(m);
}