URAL1114 Boxes 组合计数

long long C[40][40];

void get_C(int maxn)
{
    C[0][0] = 1;
    for (int i = 1; i <= maxn; i++)
    {
        C[i][0] = 1;
        for (int j = 1; j <= i; j++)
            C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
    }
}


int main() {
    ll n, a, b;
    cin >> n >> a >> b;
    ll ans = 1;
    ll c = max(a, b);
    get_C(n + c + 5);
    ll x = 1;
    ll res = 1;
    ll ans1, ans2;
    ans1 = ans2 = 1;
    for (int i = 1; i <= c; i++) {
        res += C[n++][i];
        if (i == a) ans1 = res;
        if (i == b) ans2 = res;
    }
    cout << ans1 * ans2;
}