=begin

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)?

=end

day = 1 # sunday, before jan 1, 1900
months = [31,28,31,30,31,30,31,31,30,31,30,31]
sundays = 0

1900.upto 2000 do |year|
  months.each_index do |monthi|
    monthLength = (monthi == 1 && (year % 4) == 0 && year != 1900 ? 29 : months[monthi])
    day = (day + monthLength) % 7
    sundays += 1 if day == 0 && year >= 1901
  end
end

print(sundays)