`strange = phi^2 mod n `

`phi = (p-1)(q-1) = n - (p+q) + 1`

=> `strange = (1 - (p+q))^2 = (p+q-1)^2`

Let `S = p+q -> (S-1)^2 = strange + k*n` for some small k

try small k until we get a perfect square

find k = 4

given n and S = p+q we can factor n