#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <utility>
#include <numeric>
#include <array>
#include <cmath>
#include <stack>
using namespace std;
//変数デバッグ
#define DEB(variable) cout << #variable << '=' << variable << endl

//for簡易表記(引数ミス防止)
#define FOR(LoopVariable,numberOFbegin,numberOFend) for (int LoopVariable = (numberOFbegin); (LoopVariable) < (numberOFend); (LoopVariable)++)
#define DEFOR(LoopVariable,numberOFbegin,numberOFend) for (int LoopVariable = (numberOFbegin)-1; (LoopVariable) >= (numberOFend); (LoopVariable)--)
#define REP(LoopVariable,numberOFend) for(int LoopVariable = 0;(LoopVariable)<(numberOFend);LoopVariable++)

constexpr long long int linf = numeric_limits<long long int>::max()-5;
constexpr int inf= numeric_limits<int>::max()-5;
constexpr long long int mod = 1000000007;

int main(){

    stack<char> st;

    char tmp;
    cin >> tmp;
    st.push(tmp);
    int result = 0;
    while(cin>>tmp){

        if(!st.empty()&&tmp!=st.top()){
            result+=2;
            st.pop();
            //cout << "pop!" << endl;
        }else{
            st.push(tmp);
            //cout << "push!" << endl;
        }
    }
    cout << result << endl;


    return 0;
}
