vector<int> karastuba(vector<int> a,vector<int> b)
{
    int an = a.size(),bn = b.size();
    if(an < bn)karastuba(b,a);
    if(an == 0||bn == 0)return vector<int>();
    if(an <= 50) return mutiply(a,b);
    int half=an/2;
    vector<int> a1(a.begin(),a.begin() + half);
    if(bn > half)bn=half;
    vector<int> b1(b.begin(),b.begin()+ bn);
    vector<int> a0(a.begin() + half,a.end());
    vector<int> b0(b.begin() + bn , b.end());
    vector<int> z2 = karastuba(a1 , b1);
    vector<int> z0 = karastuba(a0, b0);
    addto(a0,a1,0);
    addto(b0,b1,0);
    vector<int> z1 = karastuba(a0,b0);
    subfrom(z1,z0);
    subfrom(z1,z2);
    vector<int> ret;
    addto(ret,z2,half+half);
    addto(ret,z1,half);
    addto(ret,z0,0);
    return ret;
}