projecteuler.net/002/main.rs

fn main() {
    let mut a1 = 1;
    let mut a2 = 2;
    let mut sum = 0;

    while a2 <= 4_000_000 {
        if a2 % 2 == 0 {
            sum += a2;  
        }
        let a_new = a1 + a2;
        a1 = a2;
        a2 = a_new;
        // println!("{}", a_new);
    }
    println!("{}", sum);
}