#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;

#define ll long long
typedef tree<ll,null_type,less<ll>,rb_tree_tag,
tree_order_statistics_node_update> indexed_set;
#define ld long double
#define fi first
#define endl "\n"
#define se second
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define loop(i,a,b) for(ll i=a;i<b;i++)
#define initialize(array,size,value) for(ll i=0;i<size;i++) array[i]=value
#define couta(array,size) for(ll i=0;i<size;i++) cout<<array[i]<<" "
#define debug(x) cout<<"x: "<<x<<endl
#define dbug(x,y) cout<<"x: "<<x<<"    "<<"y: "<<y<<endl
#define inf (long long int) 1e18
#define eps 0.000001
#define vl vector<ll> 
#define sl set<ll>
#define pll pair<ll, ll>
#define mll map<ll, ll> 
#define pq priority_queue<ll>
#define mod 1000000007
#define MAXN 100001
ll spf[MAXN]; 
ll gcd(ll a,ll b);
ll palindrome(string s);
ll modexp(ll a,ll b,ll m);
void sieve();
ll ceil(ll a, ll b);
vl getFactorization(ll x);
void getZarr(string str, ll Z[]);
vector<ll> prefix_function(string s);

ll fac(ll n){
    ll ans=1;
    loop(i,1,n+1){
        ans=ans*(i);
    }
}

ll f(ll n, ll r){
    cout<<fac(n)<<" "<<fac(n-r)<<" "<<fac(r)<<endl;
    return (fac(n)/(fac(n-r)* fac(r)));
}

int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    /*#ifndef ONLINE_JUDGE
        freopen("input.txt", "r", stdin);
        freopen("output.txt", "w", stdout);
    #endif*/

    string s;
    cin>>s;
    ll n=s.length();
    stack<char> st;
    loop(i,0,n){
        if(st.empty()==1){
            st.push(s[i]);
        }
        else {
            if((s[i]=='1' && st.top()=='0') || (s[i]=='0' && st.top()=='1')){
                st.pop();
            }
            else {
                st.push(s[i]);
            }
        }
    }

    cout<<n-st.size()<<endl;
}

ll gcd(ll a, ll b){
    if (a == 0)   return b;
    if(b==0)      return a;
    if (a == b)   return a;
    if (a > b)    return gcd(a%b, b);
    return gcd(a, b%a);
}

ll palindrome(string s){
    ll l = 0;
    ll h = s.length() - 1;
    while (h > l){
        if (s[l++] != s[h--]){
            return 0;
        }
    }
    return 1;
}

ll modexp(ll a, ll b, ll m){
    if (b==0)
        return 1;
    ll temp = modexp(a,b/2,m);
    temp = (temp*temp)%m;
    if (b&1)
        return (temp*(a%m))%m; // if b is odd a^b = a^(b/2)*a^(b/2)*a
    return temp;
}

void sieve() { 
    spf[1] = 1; 
    for (ll i=2; i<MAXN; i++) 
        spf[i] = i; 
    for (ll i=4; i<MAXN; i+=2) 
        spf[i] = 2; 
  
    for (ll i=3; i*i<MAXN; i++) { 
        if (spf[i] == i) { 
            for (ll j=i*i; j<MAXN; j+=i) 
                if (spf[j]==j) 
                    spf[j] = i; 
        } 
    } 
} 

vl getFactorization(ll x) 
{ 
    vl ret; 
    while (x != 1) 
    { 
        ret.push_back(spf[x]); 
        x = x / spf[x]; 
    } 
    return ret; 
} 

ll ceil(ll a,ll b){
    return a/b+(a%b!=0);
}

void getZarr(string str, ll Z[]){
    ll n = str.length();
    ll L, R, k;
    L = R = 0;
    for (ll i = 1; i < n; ++i){
        if (i > R){
            L = R = i;
            while (R<n && str[R-L] == str[R])
                R++;
            Z[i] = R-L;
            R--;
        } else {
            k = i-L;
            if (Z[k] < R-i+1)
                Z[i] = Z[k];
            else {
                L = i;
                while (R<n && str[R-L] == str[R])
                    R++;
                Z[i] = R-L;
                R--;
            }
        }
    }
}

vector<ll> prefix_function(string s) {
    ll n = (ll)s.length();
    vector<ll> pi(n);
    for (ll i = 1; i < n; i++) {
        ll j = pi[i-1];
        while (j > 0 && s[i] != s[j])
            j = pi[j-1];
        if (s[i] == s[j])
            j++;
        pi[i] = j;
    }
    return pi;
}


Measured memory_bytes: 512