test 4 has only nines as sum and the last one is ten so the previous nines become zeroes.

and the test1?
it should not be 4750 right

4
0 4
4 2
6 8
3 7

-> 4750 alright...

what's test1,i really want to know

that was test 1 :)

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    long i,former,latter,n,c,count9=0;
    vector<long> a(0);
    cin>>n;
    for(i=0;i<2*n;i++)
    {
        cin>>c;
        a.push_back(c);
    }
    former=a[0]+a[1];
    for(i=0;i<2*n;i+=2)
    {
        latter=a[i+2]+a[i+3];
        if(n>=2)
        {
            if(latter<10)
            {
                if(latter==9)
                {
                       count9++;
                    if(i+3==2*n-1)
                    {
                        cout<<former;
                        while(count9--)
                           cout<<9;
                    }
                }
                else if(latter!=9&&count9==0)
                {
                    cout<<former;
                    former=latter;
                }
                else if(latter!=9&&count9>0)
                {
                    cout<<former;
                    while(count9--)
                       cout<<9;
                    cout<<latter;
                }
            }
            else
            {
                if(count9)
                {
                    if(former==a[0]+a[1])
                        cout<<(former+1)%10;
                    else
                       cout<<former+1;
                    while(count9--)
                       cout<<0;
                    cout<<latter%10;
                }
                else
                {
                       cout<<former+1;
                    former=latter%10;
                }
        }
        }
        else
           cout<<former<<endl;
    }
    return 0;
}

Looks very strange: your program gives different answers on 1st test on different compilers! Right answer - only if compiled with C++Builder.

First time I've faced it when the student on the olympiad compiled his text with Visual C++ 6.0, and Delta Checker (my system of checking) - with C++ Builder 6.0. The answers were different. More over, this things may happen in the following cases:
- different compilers
- different operating systems (i.e. W98 and W2000)
- the hidden error in program - i.e. some kind of access violation or overflow - take it also into consideration...

ft...it's really hard to imagine...
im not very familiar with such usage "vector<long> a(0);"
so wondering whether it's correct though it looks correct in my local VC++6.0
i know this sort of datas is not legal right
4
1 9
5 5
4 3
1 3

4
5 4
4 5
4 5
5 5
is it output 10000?
but i also get WA at test4!
help me

Is it form the first or the last?
9000+1000=0000 or 1000??
Tell me ! And what was the reason of Crash?