var lines = []; var result = 0; var ans = 0;
var readline = require('readline');

var rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout,
});

rl.on('line', function (x) {
    lines.push(x);
})

rl.on('close', function () {
    var N = Number(lines[0]);
    var S = lines[1].split("");

    for (var i = 0; i < N; i++) {
        var alphabet_before = Array(26).fill(0)
        var alphabet_after = Array(26).fill(0)

        for (var j = 0; j < i; j++) { alphabet_before[S[j].charCodeAt(0) - 97]++ }
        for (var j = i; j < N; j++) { alphabet_after[S[j].charCodeAt(0) - 97]++ }

        for (var j = 0; j < 26; j++) {
            if (0 < alphabet_before[j] && 0 < alphabet_after[j]) { result++; }
        }
        ans = Math.max(ans, result);
        result = 0;
    }
    console.log(ans);
});