codeforces 1005D（数学）

#include<bits/stdc++.h>
#define x first
#define y second
#define PB push_back
#define mst(x,a) memset(x,a,sizeof(x))
#define all(a) begin(a),end(a)
#define rep(x,l,u) for(ll x=l;x<u;x++)
#define rrep(x,l,u) for(ll x=l;x>=u;x--)
#define IOS ios::sync_with_stdio(false);cin.tie(0);
using namespace std;
typedef unsigned long long ull;
typedef pair<int,int> PII;
typedef pair<long,long> PLL;
typedef pair<char,char> PCC;
typedef long long ll;
const int N=1005;
const int M=1e6+10;
const int INF=0x3f3f3f3f;
const int MOD=1e9+7;
void solve(){
    string s;cin>>s;
    int pre_sum=0,cnt=0,ans=0;
    int n=s.size();
    rep(i,0,n){
        int u=(s[i]-'0')%3;
        cnt++;
        pre_sum+=u;
        if(u%3==0 || cnt==3 || pre_sum%3==0){
            cnt=0;
            pre_sum=0;
            ans++;
        }
    }
    cout<<ans<<endl;
}
int main(){
    IOS;
    solve();
    return 0;
}