PEBBLES

#include<stdio.h>
#include<iostream>
#include<vector>
#include<csignal>
#include<algorithm>
#include<string>
#include<cstring>
#include<vector>
#include<math.h>
#include<map>
#define FOR(i,n) for(int (i)=0;(i)<(n);(i)++)
#define REP(i,a,b) for(int (i)=(a);(i)<(b);(i)++)
#define REV(i,n) for(int (i)=(n)-1;(i)>=0;(i)--)
#define GI ({int t;scanf("%d",&t);t;})
#define GL ({int64 t;scanf("%ld",&t);t;})
#define PB push_back
typedef long long int int64;
using namespace std;

int main(){
    char s[100000];
    while(scanf("%s",s)) {
        int l = strlen(s), start=-1, last=-1;
        int count[100000]={0};
        FOR(i,l) if(s[i]=='1') { start=i; last=1; count[i]=1; break; }
        if(start == -1) {
            cout<<0<<endl;
            continue;
        }
        REP(i,start,l) {
            bool odd = 1==count[i-1]%2, zero = '0'==s[i];
            count[i] = count[i-1] + (((odd && zero) || (!odd && !zero)) ? 1 : 0);
            last=count[i];
        }
        printf("%d\n",last);
    }
    return 0;
}