source_code: import java.util.*;
  
  //201920181
  
  public class Polycarp{
  
  	public static void main(String args[]){
  		Scanner s = new Scanner(System.in);
          int rem[] = new int[3];
  		
  		Arrays.fill(rem,-1);
  		rem[0] = 0;
          char ch[] = s.next().toCharArray();
  		int n = ch.length;
   		long dp[] = new long[n];
  
          int sum = 0;
  		
          for(int i=0;i<ch.length;i++){
  		   sum = sum + (ch[i]-48);
  		   if(rem[sum%3] != -1)
  			    if(i>0){
  					dp[i] = Math.max(dp[i-1],dp[rem[sum%3]]+1);}
  				else
  					dp[i] = 1;
  	       else
  			   if(i>0)
  				   dp[i] = dp[i-1];
  			   
  		   
  		   rem[sum%3] = i;
  		   sum = sum%3;
  		   
  		}		
  		 
  
              System.out.println(dp[n-1]);			
  		
  		
  	}
  }
  