constauto& g =gcd(dx, dy);

int a = dx / g, b = dy / g;

if (a <0 || (a ==0 && b <0)) {

a = -a;

b = -b;

a = -a, b = -b;

}

constint c = b * x1 - a * y1;

result += lookup_slope[tuple(a, b)]++ - lookup_line[tuple(a, b, c)]++;