#pragma once
#include <stdio.h>
#include <sstream>
#include <string.h>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <deque>
#include <limits>
#include <iomanip>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
#define bit(x,v) ((ll)x << v)

const ll INF = 1000000007;
const ll MAX = 210000;
const ll MOD = 1000000007;

long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
	fac[0] = fac[1] = 1;
	finv[0] = finv[1] = 1;
	inv[1] = 1;
	for (int i = 2; i < MAX; i++) {
		fac[i] = fac[i - 1] * i % MOD;
		inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
		finv[i] = finv[i - 1] * inv[i] % MOD;
	}
}
ll gcd(ll a, ll b) {
	if (b == 0) return a;
	return gcd(b, a % b);
}
int pr[100010];
void uini(int n) {
	for (size_t i = 0; i <= n; i++)
	{
		pr[i] = i;
	}
}

int parent(int x) {
	if (x == pr[x]) return x;
	return pr[x] = parent(pr[x]);
}

bool unit(int x, int y) {
	int px = parent(x);
	int py = parent(y);

	if (px == py) return false;
	if (px < py) {
		pr[py] = px;
	}
	else {
		pr[px] = py;
	}
	return true;
}

// res[i][c] := i 文字目以降で最初に文字 c が登場する index (存在しないときは n)
vector<vector<int> > calcNext(const string & S) {
	int n = (int)S.size();
	vector<vector<int> > res(n + 1, vector<int>(26, n));
	for (int i = n - 1; i >= 0; --i) {
		for (int j = 0; j < 26; ++j) res[i][j] = res[i + 1][j];
		res[i][S[i] - 'a'] = i;
	}
	return res;
}
// mod 1000000007 の世界で a += b する関数
void add(long long& a, long long b) {
	a += b;
	if (a >= MOD) a -= MOD;
}
ll slide(vector<ll> & v, ll a, ll b) {
	ll azv = abs(v[0] - a);
	ll bzv = abs(v[0] - b);
	ll aov = abs(v[1] - a);
	ll bov = abs(v[1] - b);
	v[0] = a;
	v[1] = b;
	return max((azv + bov), (bzv + aov));
}


bool a[100010];
string gcs(string s, string t) {

	ll high = min(s.size(), t.size());
	ll low = 0;
	ll mid = -1;
	for (size_t i = 0; i < 20; i++)
	{
		mid = (high + low) / 2;
		string st = s.substr(0, mid);
		string tt = t.substr(0, mid);
		if (st == tt) {
			low = mid;
		}
		else high = mid;

		if (s.size() <= mid || t.size() <= mid) continue;
		st = s.substr(0, mid + 1);
		tt = t.substr(0, mid + 1);
		if (st == tt) {
			low = mid + 1;
			mid = mid + 1;
		}
	}
	return s.substr(0, mid);
}

ll dp[100010][6];
int pi[100010][6];
ll dist[7][7];

ll calc(ll now, ll b, int c, int p, int v) {

	if (b < 0) return INF;
	ll dis = now - b - 1LL;

	ll val = dp[b][p] + max(dis, dist[p][c]);
	if (v > val) {
		val = min(val, calc(now, pi[b][p], c, p, val));
	}
	return val;
}

struct bridge {
	int l, r;
};
bridge bs[100010];
bool cm(bridge& left, bridge& right) {
	return left.r < right.r;
}
struct edge {
	int t;
	ll cost;
	edge(int t, ll cost) : t(t), cost(cost) {}
};
vector<edge> es[100010];
int calc(int a, int b) {
	int rem = 24 - a;
	int tb = (b + rem) % 24;
	if (tb == 0) tb = 24;
	return min(tb, 24 - tb);

}

void solv() {
	
	string s;
	cin >> s;
	int n = s.size();
	int res = 0;
	string ts = s;
	if (ts[0] == '1') {
		ts[0] = '0';
		res++;
	}
	for (size_t i = 1; i < n; i++)
	{
		if (ts[i] == ts[i - 1]) {
			res++;
			ts[i] = ts[i - 1] == '0' ? '1' : '0';
		}
	}
	int res2 = 0;
	string tbs = s;
	if (tbs[0] == '0') {
		tbs[0] = '1';
		res2++;
	}
	for (size_t i = 1; i < n; i++)
	{
		if (tbs[i] == tbs[i - 1]) {
			res2++;
			tbs[i] = tbs[i - 1] == '0' ? '1' : '0';
		}
	}
	cout << min(res, res2) << endl;

}

int main() {
	//COMinit();
	solv();

	return 0;
}


Measured memory_bytes: 4608