I think my program is correct. Who can tell me why I got wa?

#include<cstdio>
#include<cstdlib>
#include<cstring>
char a[100005],b[100000];
char cs[]="=+-*/0123456789)(";
bool pd(char ch){
    for(int i=0;i<17;i++)
       if(ch==cs[i]) return 1;
    return 0;
}

int main(){
    while(gets(b)){
       strcat(a,b);
    }
    int len=strlen(a);
    int ha=0,k=0;
    for(int i=0;i<len;i++){
         if(ha){
            if(a[i]=='*' && a[i+1]==')')
                ha=0;
         }else{
            if(a[i]=='*' && a[i-1]=='(')
                ha=1;
            else{
                if(k>0 && (!pd(a[i]))){ printf("NO"); return 0; }
                if(a[i]=='(') k++;
                if(a[i]==')') k--;
                if(k<0) { printf("NO"); return 0;}
            }
         }
    }
    if(ha || k!=0) { printf("NO"); return 0; }
    else printf("YES");
    system("pause");
    return 0;
}