为什么RE？求助

#include <bits/stdc++.h>
using namespace std;
int qian[50001];
int hou[50001];
int zhong[50001];

int main()
{
    string a1;
    string b2;
    scanf("%s%s",&a1,&b2);
    //string zhong;
    int ch1=a1.length();
    int ch2=b2.length();
    int jinwei;
    for( int i=ch1-1;i>=0;i--)
    {
        qian[ch1-i]=a1[i]-'0';
    }
    for( int i=ch2-1;i>=0;i--)
    {
        hou[ch2-i]=b2[i]-'0';

    }

    for( int i=1;i<=ch1;i++)
    {
        for( int j=1;j<=ch2;j++)
        {
            zhong[i*j]+=(qian[i]*hou[j]);
            jinwei=zhong[i*j]/10;
            zhong[i*j]%=10;
            zhong[i*j+1]+=jinwei;
        }
    }
    int ch3=ch1*ch2+2;
    while(zhong[ch3]==0) ch3--;

    for( int i=ch3;i>=1;i--)
    {
        cout<<zhong[i];
    }
   // return;
    return 0;
}