import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readV(T...)(ref T t){auto r=rdsp;foreach(ref v;t)pick(r,v);}

void main()
{
  int n; readV(n);
  string s; readV(s);

  auto t = new int[](n);
  foreach (i; 0..n) t[i] = cast(int)(s[i]-'a');

  auto calc(int[] a, int[] b)
  {
    return setIntersection(a.dup.sort().uniq, b.dup.sort().uniq).walkLength.to!int;
  }

  auto m = 0;
  foreach (i; 1..n-1)
    m = max(m, calc(t[0..i], t[i..$]));

  writeln(m);
}
