/*
 *    J1K7_7
 *
 *    You can use my contents on a Creative Commons - Attribution 4.0 International - CC BY 4.0 License.  XD 
 *    - JASKAMAL KAINTH
 */
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <cstring>
#include <cassert>
#include <list>
#include <map>
#include <unordered_map>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <utility>
#include <bitset>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <limits>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<long long> vll;
#define left(x) (x << 1)
#define right(x) (x << 1) + 1
#define mid(l, r) ((l + r) >> 1)
#define mp make_pair
#define pb push_back
#define all(a) a.begin(), a.end()
#define debug(x)                          \
    {                                     \
        cerr << #x << " = " << x << "\n"; \
    }
#define debug2(x, y)                                                  \
    {                                                                 \
        cerr << #x << " = " << x << ", " << #y << " = " << y << "\n"; \
    }
#define debug3(x, y, z)                                                                           \
    {                                                                                             \
        cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << "\n"; \
    }
#define debug4(x, y, z, w)                                                                                                    \
    {                                                                                                                         \
        cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << ", " << #w << " = " << w << "\n"; \
    }
#define ss second
#define ff first
#define m0(x) memset(x, 0, sizeof(x))

inline int nextint()
{
    int x;
    scanf("%d", &x);
    return x;
}
inline ll nextll()
{
    ll x;
    scanf("%lld", &x);
    return x;
}

#define gc getchar
template <typename T>
void scanint(T &x)
{
    T c = gc();
    while (((c < 48) || (c > 57)) && (c != '-'))
        c = gc();
    bool neg = false;
    if (c == '-')
        neg = true;
    x = 0;
    for (; c < 48 || c > 57; c = gc())
        ;
    for (; c > 47 && c < 58; c = gc())
        x = (x * 10) + (c - 48);
    if (neg)
        x = -x;
}
// variadics
template <typename T>
T min_(T a, T b) { return a > b ? b : a; }
template <typename T, typename... Ts>
T min_(T first, Ts... last) { return min_(first, min_(last...)); }

// lambda exp auto  square = [](int inp) { return inp * inp; } ;

template <class T, class S>
std::ostream &operator<<(std::ostream &os, const std::pair<T, S> &t)
{
    os << "(" << t.first << ", " << t.second << ")";
    return os;
}
template <typename T>
ostream &operator<<(ostream &out, const vector<T> &v)
{
    out << "[";
    size_t last = v.size() - 1;
    for (size_t i = 0; i < v.size(); ++i)
    {
        out << v[i];
        if (i != last)
            out << ", ";
    }
    out << "]";
    return out;
}

ll pwr(ll base, ll p, ll mod)
{
    ll ans = 1;
    while (p)
    {
        if (p & 1)
            ans = (ans * base) % mod;
        base = (base * base) % mod;
        p /= 2;
    }
    return ans;
}
ll gcd(ll a, ll b) { return b == 0 ? a : gcd(b, a % b); }
ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

const long double PI = (long double)(3.1415926535897932384626433832795);
const ll mx_ll = numeric_limits<ll>::max();
const int mx_int = numeric_limits<int>::max();
const int mod = 1e9 + 7;

const int maxn = 2e5+6;
int dp[maxn];
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    //reopen("test.in", "rb", stdin);
    
    // int n; cin >> n;
    // vector<int> a; a.resize(n); 
    // for(int i = 0; i < n; i++)
    // {
    //     cin >> a[i];
    // }
    // int fans = 1;
    // for(int i = 1; i < n; i++)
    // {
    //     bool ok = true;
    //     for(int j = 0; j < i; j++)
    //     {
    //         if(a[j] > a[i])
    //         {
    //             ok = false;
    //         }
    //     }
    //     if(ok)
    //         fans += 1;
    // }

    // cout << fans << "\n";

    // int a,b; cin >> a >> b;
    // int fans = -1;
    // fans = max(2*a-1,2*b-1);
    // fans = max(fans,a+b);
    // cout << fans << "\n";

    // 0 - black
    // 1 - white

    string str; cin >> str;
    if(str.size() == 1)
    {
        cout << 0;
        return 0;
    }
    string s1,s2;
    int turn  = 0;
    int ans1 = 0;
    for (int i = 0; i < str.size(); i++)
    {
        // 010101010
        if (turn == 0)
        {
            if (str[i] != '0')
            {
                ans1 += 1;
            }
            turn = 1;
        }
        else
        {
            if (str[i] != '1')
            {
                ans1 += 1;
            }
            turn = 0;
        }
    }
    int ans2 = 0;
    turn = 1;
    for (int i = 0; i < str.size(); i++)
    {
        // 010101010
        if (turn == 0)
        {
            if (str[i] != '0')
            {
                ans2 += 1;
            }
            turn = 1;
        }
        else
        {
            if (str[i] != '1')
            {
                ans2 += 1;
            }
            turn = 0;
        }
    }
    cout << min(ans1,ans2) << "\n";
    return 0;
}


Measured memory_bytes: 512