fn marsExploration(s: &str) -> i32 {
    let temp: Vec<char> = s.chars().collect();
    let mut valid = 0;

    for (idx, c) in temp.iter().enumerate() {
        if (idx % 3 == 0 || idx % 3 == 2) && *c == 'S' {
            valid += 1;
        } else if idx % 3 == 1 && *c == 'O' {
            valid += 1;
        }
    }
    s.len() as i32 - valid
}