#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> PP;
// #define MOD 1000000007
#define MOD 998244353
#define INF 2305843009213693951
#define PI 3.141592653589
#define setdouble setprecision
#define REP(i,n) for(ll i=0;i<(n);++i)
#define OREP(i,n) for(ll i=1;i<=(n);++i)
#define RREP(i,n) for(ll i=(n)-1;i>=0;--i)
#define ORREP(i,n) for(ll i=(n);i>=1;--i)
#define rep(i,a,b) for(ll i=(a);i<=(b);++i)
#define ALL(v) (v).begin(), (v).end()
#define chmin(k,m) k = min(k,m)
#define chmax(k,m) k = max(k,m)
#define GOODBYE do { cout << "-1" << endl; return 0; } while (false)
#define MM <<" "<<
#define Endl endl


int main(void){
    //cin.tie(nullptr);
    //ios::sync_with_stdio(false);
    
    string S;
    cin >> S;
    ll N = S.size();
    ll Ans = 0;
    
    ll zero = 0;
    RREP(i,N){
      if(S[i]=='0'){
        zero++;
      }
      if(i < N-1 && S[i]=='1' && S[i+1]=='1'){
        
        // cout << i MM zero << endl;
        Ans += zero;
        if(zero >= 1)S[i] = '0';
        if(zero >= 2)S[i+1] = '0';
        
      }else if(i < N-1 && S[i]=='0' && S[i+1]=='1'){
        zero = 1;
        
      }

    }
    cout << Ans << endl;

    return 0;
    
}