Euler 019

• 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)?
Answer: 171