Problem:
Score : 300 points

Problem Statement

N tiles are arranged in a row from left to right. The initial color of each tile is represented by a string S of length N.

The i-th tile from the left is painted black if the i-th character of S is 0, and painted white if that character is 1.

You want to repaint some of the tiles black or white, so that any two adjacent tiles have different colors.

At least how many tiles need to be repainted to satisfy the condition?

Constraints

1 \leq |S| \leq 10^5

S_i is 0 or 1.

Input

Input is given from Standard Input in the following format:

S

Output

Print the minimum number of tiles that need to be repainted to satisfy the condition.

Sample Input 1

000

Sample Output 1

1

The condition can be satisfied by repainting the middle tile white.

Sample Input 2

10010010

Sample Output 2

3

Sample Input 3

0

Sample Output 3

0

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#define start_routine() int begtime = clock();
#define end_routine() int endtime = clock(); cerr << endl << "Time elapsed: " << (endtime - begtime)*1000/CLOCKS_PER_SEC << " ms"; return 0
#define speed() cin.tie(0), cout.tie(0), ios_base::sync_with_stdio(false)
// #define exit(a, b) return cout << a, b;

#define PB push_back
#define MP make_pair

#define sd(n) scanf("%lld", &n)
#define pdn(n) printf("%lld\n", n);
#define pds(n) printf("%lld ", n);
#define endl '\n'

#define forn(a, b, i) for (int i = a; i < b; i += 1)
#define all(v) v.begin(), v.end() 

#define print(stuff) cout << stuff << endl
#define len(stuff) (int) stuff.size()

#define int long long
#define float long double 
#define ll long long
#define ld long double

using vi = vector<int>;
using vb = vector<bool>;
using pii = pair<int, int>;
using mii = map<int, int>;

const int inf = (int) 1e16;
const int upper = (int) 3e5 + 50;
// const int M = (ll) 1e9 + 7;
const int mod = (ll) 998244353;
const double eps =  1e-8;

int diff (string &a, string &b) {
	int ans = 0;
	for (int i = 0; i < len(a); i++) {ans += (a[i] != b[i]);} return ans;
}

signed main () {
	start_routine();
	speed();
	#ifndef ONLINE_JUDGE
	// freopen("input.txt", "r", stdin);	 
	// freopen("output.txt", "w", stdout);
	// freopen("errlog.txt", "w", stderr);
	#endif	

	string a; cin >> a;
	int n = len(a);
	string b(n, '0'), c(n, '0');
	for (int i = 0; i < len(a); i += 1) {
		b[i] = char(i % 2 + '0');
		c[i] = char((i + 1) % 2 + '0');
	}
	cout << min(diff(a, b), diff(a, c));
 	end_routine();		
}