Create point c(2,2) in order to form a triangle.
Now you have 3 sides of the triangle
side ac (length 1)
side bc ( length 1)
and side ab (length ?)
Use pythagorean theorem which says the length of the hypotenuse of a right angled triangle is the squareroot of the sum of the squares of the other 2 sides.
square of side ac is 1^2 = 1
square of side bc is 1^2 = 1
length of ab = sqrt(1 + 1) = sqrt(2)