The long long type uses 8 bytes (64 bits) instead of 4. It stores numbers up to about 9 quintillion, which is far larger than int can hold. Use long long when calculations might exceed 2 billion.
Multiplying two large int values can overflow even if both inputs individually fit in int. Example: long long population = 8000000000; stores 8 billion. With int, this would overflow and give you a wrong negative number.