#include <bits/stdc++.h>
int A[] = {0, 0};
int main(void) {
  while (true) {
    char c = getchar();
    if (c == EOF) break;
    A[c - '0']++;
  }
  printf("%d\n", (A[0] > A[1] ? A[1] : A[0]) << 1);
  return 0;
}
