#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9, MOD = 1e9 + 7;
const long long LINF = 1e18;
long long int n, cnt = 0, ans = 0, b, c, d, cmp, cmpp, m, h, w, x, y, sum = 0,
                 pos;
int dy[] = {1, 0, -1, 0};
int dx[] = {0, 1, 0, -1};
string alph("abcdefghijklmnopqrstuvwxyz"), s;
bool fl = true;
struct edge {
  int to, cost;
};
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> s;
  stack<char> a;
  a.push('Z');
  for (long long int(i) = 0; (i) < (int)((s).size()); (i)++) {
    if (s[i] == '0') {
      if (a.top() == '1') {
        a.pop();
        cnt += 2;
      } else
        a.push(s[i]);
    } else {
      a.push(s[i]);
    }
  }
  for (long long int(i) = 0; (i) < (int)((s).size()); (i)++) {
    if (s[i] == '1') {
      if (a.top() == '0') {
        a.pop();
        ans += 2;
      } else
        a.push(s[i]);
    } else {
      a.push(s[i]);
    }
  }
  cout << (max(cnt, ans)) << endl;
  return 0;
}
