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