Overflows under certain conditions due to wrong variable type.

[Ryu0118]

A bug that causes overflow occurs.
Since the type of the variable is usize, it overflows when a negative value is entered.
It happens at line 146 of src/main.rs.

let require_width = index * 2;
let current_width = month_line.len();
let require_space = require_width - current_width;

Converting from usize to i64 solves this problem.