#Counting Sundays
#Problem 19
#
#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)?
# 

# let's encode sunday 0 - mon 1 ...
# 1 Jan 1901
a = 365 #1900 is not a leap year
a += 1  #1 jan is monday
a %= 7  #the day of 1 jan 1901
c = 0   #count of months for which sundays fell on march.
m = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ]
for i in range( 1901, 2001 ):
    for j in range( 1, 13 ):
        if a==0:
            c += 1
        a += m[j-1]
        if j==2 and i%4==0:
            a += 1
        a %= 7
print(c)

