here is the algorithm:
#include <iostream>
using namespace std;
int main() {
#ifndef ONLINE_JUDGE
   freopen("input.txt", "rt", stdin);
   freopen("output.txt", "wt", stdout);
#endif
    short *a_list,*d_list; unsigned short a_list_size, d_list_size, a_pos, d_pos;
    char flag;
    cin >> a_list_size;
    a_list = new short[a_list_size];
    for (a_pos = 0; a_pos<a_list_size; a_pos++) {
    cin >> a_list[a_pos];}
    cin >> d_list_size;
    d_list = new short[d_list_size];
    for (d_pos = 0; d_pos<d_list_size; d_pos++) {
    cin >> d_list[d_pos];}
    a_pos = d_pos = 0; flag = 'p';
    do {
        if ((a_list[a_pos]+d_list[d_pos]) > 10000) {
            if (d_pos == d_list_size) flag = 'n';
            else d_pos++;
        }
        else if ((a_list[a_pos]+d_list[d_pos]) < 10000) {
            if (a_pos == a_list_size) flag = 'n';
            else a_pos++;
        }
        else flag = 'y';
    } while (flag == 'p');
    if (flag == 'y') cout << "YES";
    else if (flag == 'n') cout << "NO";
}

what can be wrong?