static TEST:&[(u64, u64)] = &[  (2016, 2017), (2019, 2020), (1900, 1901),
                                (2000, 2001), (2800, 2801), (123456, 123456),
                                (1234, 5678), (123456, 7891011),
                                (123456789101112, 1314151617181920)];


fn leaps1(y:u64) -> u64 {
    y/4 - y/100 + (y/900)*2 + match y % 900 { 200...599 => 1, 600...899 => 2, _ => 0}
}

fn leaps(y1:u64, y2:u64) -> u64 {
    leaps1(y2 - 1) - leaps1(y1 - 1)
}

fn main() {
    for &(x, y) in TEST {
        println!("leaps({}, {}) => {}", x, y, leaps(x, y));
    }
}