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

T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

int calc(string s) {
    int ans = 0;
    int[int] d = [0: -1];
    int x = 0;
    for (int i = 0; i < s.length; i++) {
        switch (s[i]) {
        case 'A': x++; break;
        case 'B': x--; break;
        default: break;
        }

        if (x in d) {
            ans = max(ans, i - d[x]);
        }
        else {
            d[x] = i;
        }
    }
    return ans;
}

void main() {
    string s = readln.chomp;
    auto ans = calc(s);
    writeln(ans);
}