You are given the following information, but you may prefer to do some research for yourself.

- 1 Jan 1900 was a Monday.
- Thirty days has September,

April, June and November.

All the rest have thirty-one,

Saving February alone,

Which has twenty-eight, rain or shine.

And on leap years, twenty-nine.
- A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

module Problem19 =
open System
let start = new DateTime(1901, 1, 1)
let finish = new DateTime(2000, 12, 31)
let ans = start |> Seq.unfold (fun (t : DateTime) -> Some(t, t.AddMonths(1)))
|> Seq.filter (fun (t : DateTime) -> t.DayOfWeek = DayOfWeek.Sunday)
|> Seq.takeWhile (fun (t : DateTime) -> t <= finish)
|> Seq.length

Like all of the bigint project euler problems, the built in libraries make this completely trivial. Since my goal for this project is to learn F#/.NET, these trivial solutions are actually helpful.

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