poj 1126 Simply Syntax

#include <iostream>
#include <string>
using namespace std;
int main()
{
    string data;
    int i,top,list[260];
    bool tag;
    while(cin>>data)
    {
        top=data.size()-1;
        tag=1;
        for(i=0;i<=top;i++)
        {
            if(data[i]=='N')
                list[i]=1;
            else if(data[i]=='C'||data[i]=='D'||data[i]=='E'||data[i]=='I')
                list[i]=2;
            else if(data[i]>='p'&&data[i]<='z')
                list[i]=-1;
            else 
            {    
                tag=0;break;
            }
        }
        if(tag==0)
            printf("NO\n");
        else
        {
            int sum=0;
            while(top>=0)
            {
                sum+=list[top];
                if(sum>0)
                    break;
                if(list[top]>0)
                    sum--;
                top--;
            }
            if(sum==-1)
                printf("YES\n");
            else 
                printf("NO\n");
        }
    }
    return 0;
}