friend List operator+ (List &l1, List &l2) {
        List x;
        List res(l1);
        if(l1.n==l2.n) {
            for(int i=0; i<l1.n; ++i)
                res.niza[i]+=l2.niza[i];
            return res;
        } else
            cout<<"The list is empty"<<endl;
    }