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)?

p19()->
	put(months,{31,28,31,30,31,30,31,31,30,31,30,31}),
	io:format("~w~n",[p19({1,0,1900},0)]).
p19({_,_,2001},R)->R;
p19({D,M,1900},R)->
	p19(p19_next({D,M,1900}),R);
p19({0,M,Y},R)->
	p19(p19_next({0,M,Y}),R+1);
p19(Day,R)->
	p19(p19_next(Day),R).
p19_next({D,M,Y})->
	{(D+element(M+1,get(months)) + if M==1,(Y rem 4) ==0->1; true->0 end ) rem 7,(M+1) rem 12,
		Y + if M==11->1; true->0 end}.

def p19
  dm=[31,28,31,30,31,30,31,31,30,31,30,31]
  day=1
  year=1900
  month=0
  count=0
  while(year<2001)
    count+=1 if day==0 and year.between?(1901,2000)
    day+=dm[month]
    month=(month+1)%12
    day+=1 if month==2 and year%4==0 and year > 1900
    year+=1 if month==0
    day=day%7
  end
  puts count 
end