#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  const size_t size = S.size();
  vector<int> ans(2, 0);
  for (int i = 0; i < size; i++) {
    ans[S[i] - '0']++;
  }
  cout << min(ans[0], ans[1]) << endl;
  return 0;
}
