import std;

void main () {
    int N = readln.chomp.to!int;
    string S = readln.chomp;

    int[char] C;
    foreach (c; S) C[c]++;
    string X = "";
    foreach (i; 0..26) {
        X ~= C.get((i + 'A').to!char, 0).to!string;
    }

    writeln(X);
}

void read (T...) (string S, ref T args) {
    import std.conv : to;
    import std.array : split;
    auto buf = S.split;
    foreach (i, ref arg; args) {
        arg = buf[i].to!(typeof(arg));
    }
}