ACM 11063 B2-Sequence

#include <iostream>

using namespace std;

bool used[20001];

int main()
{
    int seq[100], no;
    int i, N, valid, a, b, temp;

    no = 1;
    while (cin >> N)
    {
        valid = 1;
        for (i = 0; i < 20001; i++)
            used[i] = false;
        for (i = 0; i < N; i++)
            cin >> seq[i];
        if (seq[0] < 1)
            valid = 0;
        for (i = 1; valid && i < N; i++)
            if (seq[i-1] >= seq[i])
            {
                valid = 0;
                break;
            }
        for (a = 0; valid && a < N; a++)
            for (b = a; b < N; b++)
            {
                temp = seq[a] + seq[b];
                if (!used[temp])
                    used[temp] = true;
                else
                {
                    valid = 0;
                    break;
                }
            }

        if (valid)
            cout << "Case #" << no++ << ": " << "It is a B2-Sequence." << endl << endl;
        else
            cout << "Case #" << no++ << ": " << "It is not a B2-Sequence." << endl << endl;
    }

    return 0;
}