#include <bits/stdc++.h>
const int INF = 2e9;
using namespace std;
using Graph = vector<vector<int>>;
int main() {
  string s;
  cin >> s;
  int n = s.size();
  int s1 = 0;
  int s0 = 0;
  for (long long i = 0; i < (n); ++i) {
    if (s[i] == '0')
      s0++;
    else
      s1++;
  }
  int ans = 2 * min(s1, s0);
}
