#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using vl = vector<ll>;
template <class T>
using vc = vector<T>;
template <class T>
using vvc = vector<vector<T>>;
const ll MOD = 1e9 + 7;
const ll INF = 1e16;
const ld EPS = 1e-11;
const ld PI = acos(-1.0L);
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
void init() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
}
int main() {
  init();
  ll cnt0 = 0, cnt1 = 0;
  string S;
  cin >> S;
  for (ll i = 0; i < (S.size()); i++) {
    if (S[i] == '0')
      cnt0++;
    else
      cnt1++;
  }
  cout << min(cnt0, cnt1) << endl;
}
