#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int n = s.length(), ans = 0;
  int a = 0, b = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == '1')
      a++;
    else
      b++;
  }
  ans = min(a, b);
  cout << ans << endl;
}
