#include<bits/stdc++.h>
using namespace std;


int main(){
  string s;
  cin >> s;
  int R_count = 0;
  int B_count = 0;
  for(int i = 0; i < s.size(); i++){
    if(s.[i] == '0') R_count++;
    else B_count++;
  }
  int ans = min(R_count,B_count);
  cout << ans << endl;

}
