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+--- Day 4: Passport Processing ---
+
+You arrive at the airport only to realize that you grabbed your North Pole Credentials instead of
+your passport. While these documents are extremely similar, North Pole Credentials aren't issued by
+a country and therefore aren't actually valid documentation for travel in most of the world.
+
+It seems like you're not the only one having problems, though; a very long line has formed for the
+automatic passport scanners, and the delay could upset your travel itinerary.
+
+Due to some questionable network security, you realize you might be able to solve both of these
+problems at the same time.
+
+The automatic passport scanners are slow because they're having trouble detecting which
+passports have all required fields. The expected fields are as follows:
+
+- byr (Birth Year) - iyr (Issue Year) - eyr (Expiration Year) - hgt (Height) - hcl (Hair Color) -
+ecl (Eye Color) - pid (Passport ID) - cid (Country ID)
+
+Passport data is validated in batch files (your puzzle input). Each passport is represented as a
+sequence of key:value pairs separated by spaces or newlines. Passports are separated by blank lines.
+
+Here is an example batch file containing four passports:
+
+ecl:gry pid:860033327 eyr:2020 hcl:#fffffd byr:1937 iyr:2017 cid:147 hgt:183cm
+
+iyr:2013 ecl:amb cid:350 eyr:2023 pid:028048884 hcl:#cfa07d byr:1929
+
+hcl:#ae17e1 iyr:2013 eyr:2024 ecl:brn pid:760753108 byr:1931 hgt:179cm
+
+hcl:#cfa07d eyr:2025 pid:166559648 iyr:2011 ecl:brn hgt:59in
+
+The first passport is valid - all eight fields are present. The second passport is
+invalid - it is missing hgt (the Height field).
+
+The third passport is interesting; the only missing field is cid, so it looks like data
+from North Pole Credentials, not a passport at all! Surely, nobody would mind if you made the system
+temporarily ignore missing cid fields. Treat this "passport" as valid.
+
+The fourth passport is missing two fields, cid and byr. Missing cid is fine, but missing any other
+field is not, so this passport is invalid.
+
+According to the above rules, your improved system would report 2 valid passports.
+
+Count the number of valid passports - those that have all required fields. Treat cid as
+optional. In your batch file, how many passports are valid?
+
+