// Problem : Coloring Colorfully
// Contest : Virtual Judge - AtCoder
// URL : https://vjudge.net/problem/AtCoder-abc124_c
// Memory Limit : 1048 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#include <bits/stdc++.h>
using namespace std;
// #pragma GCC target("avx,avx2,fma")
// //#pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC optimize("Ofast")  // disable
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define lll __int128;
using vi = vector<int>;
using vl = vector<ll>;
using pii = pair<int, int>;
using piii = pair<int, pair<int, int>>;
template <class T>
using V = vector<T>;
template <class T>
using VV = V<V<T>>;
#define PQ priority_queue
#define isin(mp, key) ((mp).find(key) != (mp).end())
#define pb push_back
#define fi first
#define se second
#define tos(x) string(1, x)
#define lp(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define vec(v, type, sz) vector<type> v(sz)
#define all(v) v.begin(), v.end()
#define MOD 1000000007
#define INF 2000000003
#define INFL 2000000000000000003ll
#define fast_io                       \
    ios_base::sync_with_stdio(false); \
    cin.tie(NULL);                    \
    cout.tie(NULL);
#define vii(v, n, m, val) vector<vector<int>> v(n, vector<int>(m, val))
#define viii(v, i, j, k, val)        \
    vector<vector<vector<int>>> v(i, \
                                  vector<vector<int>>(j, vector<int>(k, val)))
#define file_io                       \
    freopen("input.txt", "r", stdin); \
    freopen("output.txt", "w", stdout);
#define testc   \
    int tcc;    \
    cin >> tcc; \
    for (int tc = 1; tc <= tcc; tc++)
vector<string> vec_splitter(string s) {
    s += ',';
    vector<string> res;
    while (!s.empty()) {
        res.push_back(s.substr(0, s.find(',')));
        s = s.substr(s.find(',') + 1);
    }
    return res;
}
void debug_out(vector<string> __attribute__((unused)) args,
               __attribute__((unused)) int idx,
               __attribute__((unused)) int LINE_NUM) {
    cerr << endl;
}
template <typename Head, typename... Tail>
void debug_out(vector<string> args, int idx, int LINE_NUM, Head H, Tail... T) {
    if (idx > 0)
        cerr << ", ";
    else
        cerr << "Line(" << LINE_NUM << ") ";
    stringstream ss;
    ss << H;
    cerr << args[idx] << " = " << ss.str();
    debug_out(args, idx + 1, LINE_NUM, T...);
}
#ifdef DB
#define db(...) debug_out(vec_splitter(#__VA_ARGS__), 0, __LINE__, __VA_ARGS__)
#else
#define db(...) 42
#endif
#define rd_macro(_1, _2, _3, _4, NAME, ...) NAME
#define rd(...) rd_macro(__VA_ARGS__, rd4, rd3, rd2, rd1)(__VA_ARGS__)
#define rd1(x) \
    int x;     \
    cin >> x;
#define rd2(x, y) \
    int x, y;     \
    cin >> x >> y;
#define rd3(x, y, z) \
    int x, y, z;     \
    cin >> x >> y >> z;
#define rd4(w, x, y, z) \
    int w, x, y, z;     \
    cin >> w >> x >> y >> z;
template <typename... Args>
void pr(Args... args) {
    (std::cout << ... << args);
}
template <typename... Args>
void in(Args&&... args) {
    (std::cin >> ... >> args);
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int)v.size(); ++i) {
        os << v[i];
        if (i != (int)v.size() - 1) os << " ";
    }
    return os;
}
template <typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (int i = 0; i < (int)v.size(); ++i) {
        is >> v[i];
    }
    return is;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& v) {
    os << "[";
    for (auto it : v) {
        os << it;
        if (it != *v.rbegin()) os << ", ";
    }
    os << "]\n";
    return os;
}
template <typename T, typename S>
ostream& operator<<(ostream& os, const map<T, S>& v) {
    for (auto it : v) os << it.first << " : " << it.second << "\n";

    return os;
}
template <typename T, typename S>
ostream& operator<<(ostream& os, const pair<T, S>& v) {
    os << "(";
    os << v.first << ", " << v.second << ")";
    return os;
}
template <typename T, typename S>
istream& operator>>(istream& is, pair<T, S>& v) {
    is >> v.first >> v.second;
    return is;
}

signed main() {
    fast_io;
    string s;
    in(s);
    int ans1 = 0, ans2 = 0;
    for (int i = 0; i < (int)s.length(); i++) {
        int col = s[i] - '0';
        // 1 at odd pos
        if (i & 1)
            ans1 += col == 0;
        else
            ans1 += col == 1;
        // 1 at even pos
        if (i & 1)
            ans2 += col == 1;
        else
            ans2 += col == 0;
    }
    pr(min(ans1, ans2));
}
// code!=think
// least efficient code to get ac


Measured memory_bytes: 3736