#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long powmod(long long a, long long b, long long MOD) {
  long long ans = 1ll;
  while (b) {
    if (b & 1) ans = ans * a % MOD;
    a = a * a % MOD;
    b >>= 1;
  }
  return ans;
}
inline void getInt(int* p);
const int maxn = 1000010;
const int inf = 0x3f3f3f3f;
string x;
int main() {
  cin >> x;
  int cnt = 0;
  while (1) {
    int id = x.find("01");
    if (id == -1) {
      id = x.find("10");
      if (id == -1) {
        break;
      }
    }
    x.erase(id, 2);
    cnt += 2;
  }
  cout << cnt << endl;
  return 0;
}
inline void getInt(int* p) {
  char ch;
  do {
    ch = getchar();
  } while (ch == ' ' || ch == '\n');
  if (ch == '-') {
    *p = -(getchar() - '0');
    while ((ch = getchar()) >= '0' && ch <= '9') {
      *p = *p * 10 - ch + '0';
    }
  } else {
    *p = ch - '0';
    while ((ch = getchar()) >= '0' && ch <= '9') {
      *p = *p * 10 + ch - '0';
    }
  }
}
