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


void main() {
    int[int] a;

    string s;
    scan(s);
    int p;
    int ans;
    a[0] = -1;

    foreach (int i, ch; s) {
        if (ch == 'A') {
            p++;
        }
        else {
            p--;
        }

        if (p in a) {
            ans = max(ans, i - a[p]);
        }
        else {
            a[p] = i;
        }
    }

    writeln(ans);
}



void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}



void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}