n m - + - - 2 m n --------- n - m
m n -2 + - + - n m ---------- n - m
(-2.0 + m/n + n/m)/(n - m)
2 2 m + n - 2*m*n --------------- m*n*(n - m)
n - m ----- m*n
-(m - n) --------- m*n