#include "bits/stdc++.h"

using ll = long long;

using namespace std;
#define int ll

//0 -> a-1
#define rep(i, a) for (int i = 0; (i) < (int)(a); (i)++)
//a -> b-1
#define reps(i, a, b) for (int i = (int)(a); (i) < (int)(b); (i)++)
//a-1 -> 0
#define rrep(i, a) for (int i = (int)a - 1; (i) >= 0; (i)--)
//a-1 -> b
#define rreps(i, a, b) for (int i = (int)(a)-1; (i) >= (int)(b); (i)--)
#define MP(a, b) make_pair((a), (b))
#define PB(a) push_back((a))
#define all(v) (v).begin(), (v).end()
//next_permutation(all(v))
#define PERM(v) next_permutation(all(v))
/*sort(all(v));
(v).erase(unique(all(v)), v.end())*/
#define UNIQUE(v) \
    sort(all(v)); \
    (v).erase(unique(all(v)), v.end())
#define CIN(type, x) \
    type x;          \
    cin >> x
#define YES(f) if ((f)) {cout << "YES" << endl;} else {cout << "NO" << endl;}
#define Yes(f) if ((f)) {cout << "Yes" << endl;} else {cout << "No" << endl;}
#define MINV(v) min_element(all(v))
#define MAXV(v) max_element(all(v))
#define MIN3(a, b, c) min(min(a, b), c)
#define MIN4(a, b, c, d) min(MIN3(a, b, c), d)
#define MIN5(a, b, c, d, e) min(MIN4(a, b, c, d), e)
#define MIN6(a, b, c, d, e, f) min(MIN5(a, b, c, d, e), f)
#define MAX3(a, b, c) max(max(a, b), c)
#define MAX4(a, b, c, d) max(MAX3(a, b, c), d)
#define MAX5(a, b, c, d, e) max(MAX4(a, b, c, d), e)
#define MAX6(a, b, c, d, e, f) max(MAX5(a, b, c, d, e), f)
//b is [a, c)
#define RANGE(a, b, c) ((a) <= (b) && (b) < (c))
//c is [a, e) && d is [b, f)
#define RANGE2D(a, b, c, d, e, f) (RANGE((a), (c), (e)) && RANGE((b), (d), (f)))
#define chmin(a, b) a = min(a, (b))
#define chmin3(a, b, c) a = MIN3(a, (b), (c))
#define chmin4(a, b, c, d) a = MIN4(a, (b), (c), (d))
#define chmin5(a, b, c, d, e) a = MIN5(a, (b), (c), (d), (e))
#define chmin6(a, b, c, d, e, f) a = MIN6(a, (b), (c), (d), (e), (f))
#define chmax(a, b) a = max(a, (b))
#define chmax3(a, b, c) a = MAX3(a, (b), (c))
#define chmax4(a, b, c, d) a = MAX4(a, (b), (c), (d))
#define chmax5(a, b, c, d, e) a = MAX5(a, (b), (c), (d), (e))
#define chmax6(a, b, c, d, e, f) a = MAX6(a, (b), (c), (d), (e), (f))
#define fcout cout << fixed << setprecision(12)
#define RS resize
#define CINV(v, N)             \
    do                         \
    {                          \
        v.RS(N);               \
        rep(i, N) cin >> v[i]; \
    } while (0);

#define RCINV(v, N)             \
    do                          \
    {                           \
        v.RS(N);                \
        rrep(i, N) cin >> v[i]; \
    } while (0);
#define MOD 1000000007

template <class T>
inline T GET()
{
    T x;
    cin >> x;
    return x;
}

template<class T, char d='\n'>
struct Vector {
    vector<T> v;
    Vector(int sz) : v(sz) {}
    Vector(int sz, T t) : v(sz, t) {}
    Vector() : v(0) {}
    Vector(vector<T> t) : v(t) {}
    inline auto begin() {return v.begin();}
    inline auto end() {return v.end();}
    inline auto rbegin() {return v.rbegin();}
    inline auto rend() {return v.rend();}
    inline int size() {return v.size();}
    inline auto resize(int sz) {v.resize(sz);}
    inline auto resize(int sz, T t) {v.resize(sz, t);}
    inline auto empty() {v.empty();}
    inline auto reserve(int sz) {v.reserve(sz);}
    inline auto push_back(T t) {v.push_back(t);}
    inline auto pop_back() {v.pop_back();}
    inline auto assign(int sz, const T& t) {v.assign(sz, t);}
    inline auto& front() {return v.front();}
    inline auto& back() {return v.back();}
    inline auto insert(const auto p, const T& t) {v.insert(p, t);}
    inline auto insert(const auto p, auto f, auto l) {v.insert(p, f, l);}
    inline auto erase(const auto p) {v.erase(p);}
    inline auto erase(const auto f, const auto s) {v.erase(f, s);}
    inline auto emplace_back(auto t) {v.emplace_back(t);}
    inline auto swap(auto& f) {v.swap(f.v);}
    inline auto clear() {v.clear();}
    inline auto& operator[](size_t pos) {return v[pos];}
    inline auto operator<(const auto& a) {return v < a.v;}
    inline auto operator>(const auto& a) {return a < (*this);}
    inline auto operator>=(const auto& a) {return !((*this) < a);}
    inline auto operator<=(const auto& a) {return !(a < (*this));}
    inline auto operator==(const auto& a) {return !(a < (*this)) && !((*this) < a);}
    inline friend istream& operator>>(istream& lhs, Vector& rhs) {
        for (auto& x : rhs) lhs >> x;
        return lhs;
    }
    inline friend ostream& operator<<(ostream& lhs, Vector& rhs) {
        for (int i = 0; i + 1 < rhs.size(); i++) lhs << rhs[i] << d;
        if (rhs.size()) lhs << rhs.back();
        return lhs;
    }
};

void init();
void solve();

signed main()
{
    init();
    solve();
}

void init()
{
    string s;
    cin >> s;
    int l = s.length();
    int a = 0, b = 0;
    rep(i, l) {
        if (s[i] - '0' == i % 2) a++;
    }
    rep(i, l) {
        if (s[i] - '0' == (i+1) % 2) b++;
    }
    cout << min(a, b) << endl;
}


void solve()
{
}



Measured memory_bytes: 512