`D = [1; +infty)`
`sqrt{x - 1} - sqrt{x + 4} = -1`
`-> sqrt{x - 1} = sqrt{x + 4} - 1`
`-> (sqrt{x - 1})^2 = (sqrt{x + 4} - 1)^2`
`-> x - 1 = x + 4 - 2sqrt{x + 4} + 1`
`-> 2sqrt{x + 4} = 6`
`-> sqrt{x + 4} = 3`
`-> (sqrt{x + 4})^2 = 3^2`
`-> x + 4 = 9`
`-> x = 5 (TM)`
`-> S = {5}`