#include <bits/stdc++.h>
#include <cstdlib>
using namespace std;
#define ll long long
#define MAX 100005
#define pb push_back
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
const double PI  =3.141592653589793238463;
long long MOD = 1000000007;
vector<int>adj[100005];
int stringToInteger(string x){
    int gg=0;
    stringstream geek(x);
    geek >> gg;
    return gg;
}

ll powmod(ll a,ll b,ll m)
{
    ll ans=1;
    while(b>0){ if(b%2)
        {ans*=a; ans%=m;}
         b/=2; a=a*a; a=a%m;
        }
        return ans%m;
}
ll gcd(ll a,ll b)
{

    return __gcd(a,b);
}
ll mulmod(ll a,ll b,ll m)
{  ll ans=0;
    a %=m;
    while (b>0)
    {       if(b&1) ans = (ans + a) % m;
        a = (a * 2)%m;
        b /= 2;
    }
    return ans % m;
}

ll pow(ll a, ll b, ll MOD)
{
 ll x=1,y=a;
 while(b > 0)
 	{
 		if(b%2 == 1)
 	{
 		x=(x*y);
 		if(x>MOD) x%=MOD;
 	}
 	y = (y*y);
 	if(y>MOD) y%=MOD;
 	b /= 2;
 	}
 return x;
}

int main()
{
    string s;
    cin>>s;
    stack<char>st;
    ll ans=0;
    st.push(s[0]);
    int n=s.length();
    for(int i=0;i<n;i++)
    {
        if(!st.empty()&&s[i]!=st.top())
        { st.pop(); ans+=2;
        }
        else st.push(s[i]);
    }
    cout<<ans<<"\n";
}

