//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using lll = __int128_t;
using ld = long double;
using P = pair<ll, ll>;
using vi = vector<ll>;
using vd = vector<ld>;
using vP = vector<P>;
using vS = vector<string>;
using vvi = vector<vi>;
using vvd = vector<vd>;
using vvP = vector<vP>;
using v3i = vector<vvi>;
using v3d = vector<vvd>;
using v3P = vector<vvP>;
using v4i = vector<v3i>;
using v5i = vector<v4i>;
using v6i = vector<v5i>;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using Mint = modint1000000007;
using vm = vector<mint>;
using vM = vector<Mint>;
using vvm = vector<vm>;
using vvM = vector<vM>;
using v3m = vector<vvm>;
using v3M = vector<vvM>;
using v4m = vector<v3m>;
using v5m = vector<v4m>;
#endif
template <typename T>
using rp_Q = priority_queue<T, vector<T>, greater<T>>;
#define rrep(i, n) for (ll i = (n)-1; (i) >= 0; --(i))
#define rep(i, n) for (ll i = 0; (i) < (n); ++(i))
#define loop(i, a) for(ll i = a; true; ++(i))
#define reps(i, n) for (ll i = 1; (i) <= (n); ++(i))
#define Rep(i,a,b) for(ll i = (a); i <= (b); i++)
#define all(a) (a).begin(),(a).end()
const ll MOD = 998244353;
const ll Hash = 10000000000000061;
#define Yes(b) ((b)?cout << "Yes" << endl:cout << "No" << endl)
#define YES(b) ((b)?cout << "YES" << endl:cout << "NO" << endl)
#define Aoki(b) ((b)?cout << "Aoki" << endl:cout << "Takahashi" << endl)
#define out(x) cout << (x) << endl
#define vout(v) for(ll x : (v)) cout << x << endl
#define vin(v) rep(i, (v).size()) cin >> (v)[i]
#define ft first
#define sd second
#define vInP(v) rep(i, (v).size()) cin >> (v)[i].ft >> (v)[i].sd
bool chmin(ll& a, ll b) { return a > b ? a = b, 1 : 0; }
bool chmax(ll& a, ll b) { return a < b ? a = b, 1 : 0; }
vi s90 = { 0, 1, 0, -1 }, c90 = { 1, 0, -1, 0 };
vi s45 = { 0, 1, 1, 1, 0, -1, -1, -1 }, c45 = { 1, 1, 0, -1, -1, -1, 0, 1 };

void solve();

signed main(){
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    cout << fixed << setprecision(20);

    int t = 1;
    // cin >> t;
    rep(i, t) solve();
}

void solve(){
    string S; cin >> S;
    reverse(all(S));
    ll N = S.size();
    vi a(N); rep(i, N) a[i] = S[i] - '0';
    ll ans = 0, p = 0, P = 0;
    while(true){
        if(p > N - 3) break;
        if((a[p] == 0) && (a[p + 1] == 1) && a[p + 2] == 1){
            ans++; a[p] = 1; a[p+2] = 0;
            ll q = p - 1;
            while(q > p - 3){
                if(q < 0) break;
                if((a[q] == 0) && (a[q + 1] == 1) && a[q + 2] == 1){
                    ans++; a[q] = 1, a[q + 2] = 0;
                    p = q;
                }
                q--;
            }
        }
        p = P+1; P++;
    }
    out(ans);
}