#include "bits/stdc++.h"
using namespace std;
#define int long long
#define endl '\n'
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
	string s;
	cin>>s;
	vector<int> a(s.size());
	vector<int> b(s.size());
	for(int i=0;i<s.size();i++){
		a[i]=i%2;
		b[i]=1-i%2;
	}
	int ans1=0;
	int ans2=0;
	for(int i=0;i<s.size();i++){
		if(s[i]-'0'!=a[i])ans1++;
	}
	for(int i=0;i<s.size();i++){
		if(s[i]-'0'!=b[i])ans2++;
	}
	cout<<min(ans1,ans2)<<endl;
}

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