How many points 2

For real numbers a, m > 0.0; let F(a,m) denote the number of lattice points inside the OAB tringle with the following vertices: O(0.0,0.0) A(a,0.0) B(x_b,y_b) which is the intersection point of the following lines: L1: y=x/m L2: y=m*(x-a) For example: F(8.0,3.0)=19 For O(0.0,0.0), A(8.0,0.0), B(9.0,3.0) What is F(123456789101112,2/(sqrt(5)-1))? [My timing: 1s]