#include <cstdlib>
#include <iostream>
#include <string>
#include <time.h>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <sstream>
#include <iomanip>
#include <vector>
#include <functional>
#include <numeric>
#include <cassert>
#include <algorithm>
#include <functional>
#include <tuple>
#include <bitset>
#include <queue>
#include <map>
//#include "bits/stdc++.h"
using namespace std;
//using namespace std::vector;
#define rep(i,a,b) for(ll i=(a);i<(b);++i)
//#define M_PI 3.1415926535
#define huge 1000000007
typedef long long int ll;
ll gcd(ll a, ll b);
ll lcm(ll x, ll y);
ll bur(ll N, ll X);
std::vector<ll> enum_div(ll n);
long long comb(int n, int r);
bool IsPrime(int num);
bool dp[110][10010];
class UnionFind;

//UF木
class UnionFind {
public:
	//親
	vector<int> Parent;

	//初期状態
	UnionFind(int N) {
		Parent = vector<int>(N, -1);
	}

	int root(int A) {
		if (Parent[A] < 0) return A;
		return Parent[A] = root(Parent[A]);
	}

	//サイズ確認
	int size(int A) {
		return -Parent[root(A)];
	}

	int connect(int A, int B) {
		//root同氏をくっつける
		A = root(A);
		B = root(B);

		//結合済み
		if (A == B) {
			return false;
		}
		//大きいほうに小さいほうをくっつける
		if (size(A) < size(B)) swap(A, B);

		//Aのサイズ更新
		Parent[A] += Parent[B];
		//Bの親をAに
		Parent[B] = A;

		return true;
	}

};

int main(void) {

	ll N = 0;
	ll A = 0;
	ll B = 0;
	ll C = 0;
	ll D = 0;
	ll E = 0;
	ll M = 0;
	ll K = 0;
	
	string S, T, U[52];
	char fi[50], fv[50];
	char board[4][4];
	ll count = 0;
	double count2 = 0;

	float min = 30000000;
	ll max = 0;
	ll dist1 = 0;
	ll dist2 = 0;
	ll dist3 = 0;
	float now = 0;
	int tmax = 10000;
	int tmax2 = 0;
	//int flag = 0;
	//char temp, head, state;
	//int is = 0;
	bool start = false;
	
	bool xa[26] = {};
	bool ya[26] = {};
	cin >> N >> S;

	rep(j, 1, N) {

		rep(i, 0, N) {
			if (i < j) {
				//X
				//cout << (int)S[i] << endl;
				xa[(int)S[i] - 97] = true;

			}
			else {
				//Y
				//X
				//cout << (int)S[i] << endl;
				ya[(int)S[i] - 97] = true;
			}
		}

		rep(i, 0, 26) {
			if (xa[i] && ya[i]) {
				count++;
			}

			if (xa[i])
				xa[i] = false;

			if (ya[i])
				ya[i] = false;


		}

		
		if (max < count) {
			max = count;
		}

		count = 0;
		

		

	}
	
	cout << max << endl;
	return 0;

}


std::vector<ll> enum_div(ll n)//nの約数を列挙
{
	std::vector<ll> ret;
	for (int i = 1; i*i <= n; ++i)
	{
		if (n%i == 0)
		{
			ret.push_back(i);
			if (i != 1 && i*i != n)
			{
				ret.push_back(n / i);
			}
		}
	}
	return ret;
}
ll bur(ll N, ll X) {
	ll pan = 1;
	ll pat = 1;
	ll burg = 0;



	if (X > burg) {
		if (N == 0)
			return pat;


		burg += pan;
		burg += bur(N - 1, X);
		burg += pat;
		burg += bur(N - 1, X);
		burg += pan;


	}

	return burg;

}

ll gcd(ll a, ll b) {

	/* 自然数 a > b を確認・入替 */
	if (a<b) {
		ll tmp = a;
		a = b;
		b = tmp;
	}
	ll x = b;
	ll y = a;
	/* ユークリッドの互除法 */
	ll r = a % b;
	while (r != 0) {
		a = b;
		b = r;
		r = a % b;
	}


	return y / b*x;
}

bool IsPrime(int num)
{
	if (num < 2) return false;
	else if (num == 2) return true;
	else if (num % 2 == 0) return false; // 偶数はあらかじめ除く

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// 素数ではない
			return false;
		}
	}

	// 素数である
	return true;
}

// 最大公約数を求める関数
ll lcm(ll x, ll y)
{
	if (x == 0 || y == 0)  // 引数チェック
	{
		//fprintf(stderr, "引数エラーです。\n");
		return 0;
	}

	return (x * y / gcd(x, y));
}

//combination
long long comb(int n, int r) {
	std::vector<std::vector<long long>> v(n + 1, std::vector<long long>(n + 1, 0));
	for (int i = 0; i < v.size(); i++) {
		v[i][0] = 1;
		v[i][i] = 1;
	}
	for (int j = 1; j < v.size(); j++) {
		for (int k = 1; k < j; k++) {
			v[j][k] = (v[j - 1][k - 1] + v[j - 1][k]);
		}
	}
	return v[n][r];
}

Measured memory_bytes: 256