#pragma comment(linker, "/stack:247474112")
#pragma GCC optimize("Ofast")

#include <bits/stdc++.h>

using namespace std;

#define     pii             pair<int ,int>
#define     ff              first
#define     ss              second
#define     pb(x)           push_back(x ) 
#define     mp              make_pair
#define     sz(x)           x.size()
#define     clr(x)          x.clear()
#define     mem(x ,y)       memset(x , y , sizeof x )
#define     all(a )         a.begin() , a.end()
#define     endl            "\n"
#define     si(x )          scanf("%d", &x)
#define     sii(x,y )       scanf("%d%d", &x,&y)
#define     siii(x,y ,z)    scanf("%d%d%d ", &x,&y , &z)
#define     rep(i ,n )      for(int i = 0 ; i < n ; i++ )
#define     repi(i ,n )     for(int i = 1 ; i <= n ; i++ )
#define     forn(i ,x,n )   for(int i = x ; i <= n ; i++ )
#define     repit(i, c)     for( __typeof((c).begin()) i = (c).begin(); i != (c).end();++i )
#define     dbg(args...)    do {   cerr << #args << ' ' ;  print(args); } while(0); cerr<< endl ;

typedef long long ll  ;
typedef unsigned long long  ull ;
typedef vector<int> vi  ;

const ll  mod = (ll) 1e9 +7;
//const int inf = 1e9  ;
const long long  infL = 1e18 ;

int setbit(int n, int pos  ) { return n = n|(1<< pos) ; }
int resetbit(int n,int pos ) {  return n =  n & ~(1<<pos ); }
bool checkbit(int  n,int pos ) { return (bool ) (n&(1<<pos))  ; }

template < typename T > T mul(T x, T y   ) { x %= mod ; y %= mod ; return (x*y)%mod  ;  }
template < typename T > T add(T x, T y  ) { x %= mod ; y %= mod ; return (x+y)%mod ;  }
template < typename T > T modPow(T x, T k ,T mod ) { if(k == 0 ) return 1; if(k&1  ) { return ( (x%mod)*modPow(x,k-1, mod )%mod )%mod ; } else { T ret = modPow(x,k/2 , mod ) ; ret %= mod ; return (ret*ret )%mod; } }
template < typename T > T ext_gcd(T num1 , T num2, T &X , T &Y ) { T r2,r1, q2, q1 , x2, x1, y2 ,y1 , x,  y, r   ; x2 = 1; y2 = 0;  x1 = 0; y1 = 1; for( r2 =num1 ,r1 = num2 ;r1 != 0 ; y2 = y1, y1 = y,x2 = x1 ,x1 = x ,r2 = r1, r1 = r) {q1 = r2/r1; x = x2 - q1*x1; y = y2 - q1*y1; r = r2%r1 ; } X = x2 ; Y = y2 ; return r2 ;  }
template < typename T > T modInv( T x , T m ) {  return modPow(x,m-2 , m) ; }  // long long ret, t ;// ext_gcd( x, m ,ret ,t  ) ;// ret %= m ;// if(ret<0) ret += m  ; // return ret;}
template < typename T > void print(const T& v) {    cerr << v << ' ' ;}
template < typename T1, typename... T2 > void print( const T1& first, const T2&... rest ){ print(first); print(rest...) ;}
// debuging ... 
template < typename F, typename S >
ostream& operator << ( ostream& os, const pair< F, S > & p ) {
    return os << "[ " << p.first << ", " << p.second << " ]";
}

template < typename T >
void trace( T a[], int  n) {
    rep(i,n) cout << a[i] << ' ' ;  cout<<endl;
}

template < typename T >
ostream &operator << ( ostream & os, const vector< T > &v ) {
    os << "[ "; rep(i , sz( v)  )  os << v[i] << ' ' ; return os << " ]" ; 
}

template < typename T >
ostream &operator << ( ostream & os, const set< T > &v ) {
    os << "[ "; repit(it,  v  )  os << *it << ' ' ; return os << " ]" ; 
}


//int dx[] = {0 , 0 ,+1, -1 } ;
//int dy[] = {-1, +1,0 , 0 };
  
const int M = (int )2e6+7 ;  
const int mx = (int)700; 
//const ll N = 1e75 ;
struct tt {  int x,y   ; 
    tt(int a  ,int b  )   {x =a ; y=b;  }  
    bool operator<(tt a )const  {
        if(x==a.x)return y<a.y ;
        return x< a.x  ;
    }
} ;

bool cmp(int x,int y )  { return x>y; }

//////////////////////////// code from here  ////////////////////////////


#define yes "Shi"
#define no "Fou"

ll a[M+2]  ,cnt =0 , n ,dp[2][M+2]; 
string s; 

void solve() {   
    cin >> s; 
    dp[0][0] = (s[0]=='0'?0:1 ) ;
    dp[1][0] = (s[0]=='1'?0:1 ) ;

    repi(i, sz(s)-1) {
        dp[0][i] += (s[i]=='0'? 0:1 ) + dp[1][i-1];    
        dp[1][i] += (s[i]=='1'? 0:1 ) + dp[0][i-1];  

    }
 //   trace(dp[1] , sz(s) ) ;
 // trace(dp[0] , sz(s) ) ;
    

    cout << min(dp[0][sz(s)-1] , dp[1][sz(s)-1]  ) ;  

  //  dbg("fuck "  ); 
}

int main() {
    #ifndef ONLINE_JUDGE
        //freopen("in.txt", "r", stdin);
        //freopen("out.txt", "w", stdout);
    #endif 
     ios::sync_with_stdio(false);  cin.tie(0); cout.tie(0);
    
    int t=1;  //cin >> t; 
    while(t--) {// a+n ) ;
        solve() ;
    }




    return 0 ;

} 


 

Measured memory_bytes: 5328