キーエンスプログラミングコンテスト 2019 C Exam and Wizard

int main()
{
    cout <<setprecision(10);
    ll n ;cin >>n;
    int A[100100] , B[100100];
    rep(i,n) cin >> A[i];
    rep(i,n) cin >> B[i];

    ll sum_a = 0 , sum_b = 0;
    rep(i,n){
        sum_a += A[i];
        sum_b += B[i];
    }

    vector<ll> amari(0);
    ll yowaihito = 0;
    ll need = 0;
    rep(i,n){
        if(A[i] > B[i]){
            amari.push_back(A[i]-B[i]);
        }
        else if ( B[i] > A[i]) {
            yowaihito ++;
            need += B[i] - A[i];
        }

    }

    SORT(amari);
    rSORT(amari);

    if(sum_a < sum_b){
        cout << -1 <<endl;
        return 0;
    }
    else{
        //他では、最大でnだね。

        if(yowaihito==0){
            cout << 0 << endl;
            return 0 ;
        }
        ll count = 0;
        ll yasashi = amari[count];
        while(yasashi < need){
            count ++ ;
            yasashi += amari[count];
            //count + 1 個優しい人の力を借りる
            //足りてない人の数を足せば答え
        }
        cout << yowaihito + count+1  << endl;
        return 0 ;
    }
}