#include <bits/stdc++.h>
int main(void) {
  char S[1000100];
  scanf("%s", S);
  int i, j, N = 0;
  int Nei[1000100];
  for (i = 0; S[i] != '\0'; i++) {
    N++;
    Nei[i] = i + 1;
  }
  int ans = 0;
  int flag = 0;
  int nei;
  int i1, i2;
  int count = 0;
  while (flag == 0) {
    flag = 1;
    count = 0;
    for (i = 0; i < N - 1; i++) {
      i1 = i;
      i2 = Nei[i];
      if ((S[i1] == '0' && S[i2] == '1') || (S[i1] == '1' && S[i2] == '0')) {
        ans = ans + 2;
        S[i1] = '2';
        S[i2] = '2';
        Nei[count] = i1 + 2;
        count++;
        flag = 0;
      } else if (S[i1] == '2') {
        Nei[count] = i1 + 1;
        count++;
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
