求约数和的和

//小G定义了两个函数F(n)为n的约数和，G(n)为F(1)+F(2)+...+F(n-1)+F(n)

ll G(ll x){
    ll res=0;
    for(ll i=1;i<=x;) {
        ll r = x/(x/i);
        res += (x/i) * (r + i) * (r - i + 1) / 2 ;
        i=r+1;
    }
    return res;
}