#include<bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
template<typename F, typename S>
ostream &operator<<(ostream &os, const pair<F, S> &p) {
    return os << "(" << p.first << ", " << p.second << ")";
}
template<typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "{";
    typename vector<T>::const_iterator it;
    for (it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) os << ", ";
        os << *it;
    }
    return os << "}";
}
template<typename T>
ostream &operator<<(ostream &os, const set<T> &v) {
    os << "[";
    typename set<T>::const_iterator it;
    for (it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) os << ", ";
        os << *it;
    }
    return os << "]";
}
template<typename F, typename S>
ostream &operator<<(ostream &os, const map<F, S> &v) {
    os << "[";
    typename map<F, S>::const_iterator it;
    for (it = v.begin(); it != v.end(); it++) {
        if (it != v.begin()) os << ", ";
        os << it->first << " = " << it->second;
    }
    return os << "]";
}
#define debug(x) cerr << #x << " = " << x << endl;
#define trace1(x)                cerr<<#x<<": "<<x<<endl
#define trace2(x, y)             cerr<<#x<<": "<<x<<" | "<<#y<<": "<<y<<endl
#define trace3(x, y, z)          cerr<<#x<<":" <<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl
#define trace4(a, b, c, d)       cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<endl
#define trace5(a, b, c, d, e)    cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<endl
#define trace6(a, b, c, d, e, f) cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<" | "<<#f<<": "<<f<<endl


typedef long long int ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<long long int> vll;
typedef pair<int, int> pii;
typedef pair<long long int, long long int> pll;
typedef map<int, int> mii;
typedef vector< pair<int, int> > vpii;

#define endl "\n";
#define IOS ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define pb push_back
#define mp make_pair
#define all(c) (c).begin(),(c).end()
#define tr(cont, it) for(decltype((cont).begin()) it = (cont).begin(); it != (cont).end(); it++)
#define present(c, x) ((c).find(x) != (c).end())
#define cpresent(c, x) (find(all(c),x) != (c).end())
#define F first
#define S second


const ld PI=acos(-1.0);
const ll mod = 1000000007;
const ll inf = (ll) 1e15;

ll power(ll a, ll b, ll m = mod) {if (b == 0) return 1; if (b == 1) return (a % m); ll x = power(a, b / 2, m); x = (x * x) % m; if (b % 2) x = (x * a) % m;return x;}
ll max(ll a, ll b) { return (a > b ? a : b); }
ll min(ll a, ll b) { return (a < b ? a : b); }

const int N=1e5+5;
const int M=1e5+5;

int checkdiff(string s,string s1)
{
    int ret=0;
    for(int i=0;i<s.size();i++)
    {
        if(s[i]==s1[i])
            continue;
        ret++;
    }
    return ret;
}

void solve() {

    int n;
    string s;
    cin >> s;
    n=s.size();
    string s1,s2;
    for(int i=0;i<n;i++)
    {
        if(i%2)
        {
            s1+='0';
            s2+='1';
        } else
        {
            s1+='1';
            s2+='0';
        }
    }
    cout << min(checkdiff(s,s1),checkdiff(s,s2)) << endl;

}

int main() {
    IOS;
    int t = 1, num = 1;   ///// change this t for number of testcase globally
    while (t--) {
        solve();
    }
    return 0;
}

/* stuff you should look for
    * int overflow, array bounds
    * special cases (n=1?), set tle
    * do smth instead of nothing and stay organized
*/

Measured memory_bytes: 1104