#include <iostream>
#include <string>
using namespace std;

int main(){
    string s;
    cin >> s;

    int n, r, b;
    n = s.size();
    r = 0; b = 0;
    for (int i = 0; i < n; i++){
        if (s[i] == '0') r += 1;
        else b += 1;
    }
    cout << 2 * fmin(r, b) << endl;
}
