// what is matter? never mind. 
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,sse4,popcnt,abm,mmx,avx,avx2") 
#include<bits/stdc++.h>
#define For(i,a,b) for(int i=(a);i<=(b);++i)
#define Rep(i,a,b) for(int i=(a);i>=(b);--i)
#define ll long long
#define ull unsigned long long
using namespace std;
inline int read()
{
    char c=getchar();int x=0;bool f=0;
    for(;!isdigit(c);c=getchar())f^=!(c^45);
    for(;isdigit(c);c=getchar())x=(x<<1)+(x<<3)+(c^48);
    if(f)x=-x;return x;
}

#define fi first
#define se second
#define pb push_back
#define mkp make_pair
typedef pair<int,int>pii;
typedef vector<int>vi;

#define maxn 5000055
#define inf 0x3f3f3f3f

/*
0 : R
1 : L
*/
int n;
ll res,val[maxn];
char s[maxn];
pii st[maxn];
int m;
void add(int x,int y){
//	cout<<"add "<<x<<" "<<y<<"\n";
	if(m && st[m].se==0 && y==1 && (x-st[m].fi)%4==0){
		res+=(x-st[m].fi)/4;
		--m;
		return;
	}
	st[++m]=mkp(x,y);
}

struct node{
	int l,r,op,c;
}a[maxn];
ll f[2],pos[2],g[2];

signed main()
{
	scanf("%s",s+1);n=strlen(s+1);
	add(-1,1);
	if(s[1]=='1')add(0,1);
	For(i,2,n)
		if(s[i-1]==s[i])add(i-1,s[i]&1);
	if(s[n]=='0')add(n,0);
	add(n+1,0);
//	For(i,1,m)cout<<st[i].fi<<" "<<st[i].se<<"\n";
	Rep(i,m-1,1)
		if(st[i].se==0 && st[i+1].se==0){
			int d=(st[i+1].fi-st[i].fi)/4;
			res+=d;
			st[i].fi+=d*4;
		}
	For(i,1,m-1)
		if(st[i].se==1 && st[i+1].se==1){
			int d=(st[i+1].fi-st[i].fi)/4;
			res+=d;
			st[i+1].fi-=d*4;
		}
//	cout<<"res "<<res<<"\n";
	For(i,6,n+2) val[i]=val[(i-2)/4*4]+(i-2)/4;
	int m2=0;
	for(int l=1,r;l<=m;l=r+1){
		r=l;
		while(r+1<=m && st[r+1].se==st[l].se)++r;
		a[++m2]={st[l].fi,st[r].fi,st[l].se,r-l+1};
	}
	m=m2;
	pos[0]=pos[1]=a[1].r;
//	For(i,1,m)cout<<a[i].l<<" "<<a[i].r<<" "<<a[i].op<<"\n";
//	cout<<"laspos "<<pos[0]<<" "<<pos[1]<<"\n";
	for(int i=2;i<m;i+=2){
		int t=(a[i+1].l-a[i].r)/4;
		g[0]=g[1]=-inf;
		For(j,0,1) g[0]=max(g[0],f[j]+val[a[i].l+t*4-pos[j]]);
		g[0]+=t*a[i].c;
		For(j,0,1) g[1]=max(g[1],f[j]+val[a[i].l-pos[j]]);
		g[1]+=t*a[i+1].c;
		f[0]=g[0];
		f[1]=g[1];
		pos[0]=a[i+1].r;
		pos[1]=a[i+1].r-t*4;
	//	cout<<"f: "<<f[0]<<" "<<f[1]<<"\n";
	//	cout<<"pos: "<<pos[0]<<" "<<pos[1]<<"\n";
	}
	For(i,0,1) f[i]+=val[a[m].l-pos[i]];
	res+=max(f[0],f[1]);
	cout<<res;
    return 0;
}
/*
0000010101100110101101010110000110100111000010010111111100101101110101000111101111010101010010101010

*/