use std::collections::HashSet;
use std::collections::VecDeque;

type Str = VecDeque<u8>;
type Res = HashSet<u32>;

fn pr(s: &Str) {
    for c in s.iter() { print!("{}", c); }
    println!("");
}

fn is_prime(k: u32) -> bool {
    if k < 0 {
        return false;
    }
    for i in 2..k {
        if k % i == 0 {
            return false;
        }
    }
    return true;
}

fn nstr(n: u32) -> Str {
    let mut res = Str::new();
    let mut nn = n;
    while nn > 0 {
        res.push_front((nn % 10) as u8);
        nn /= 10;
    }
    res
}


fn strn(s: &Str) -> u32 {
    let mut res = 0;
    for d in s.iter() {
        res *= 10;
        res += *d as u32;
    }
    res
}


fn test(s: &Str, hs: &mut Res) {
    let n = strn(s);
    if is_prime(n) {
        //println!("{}", n);
        hs.insert(n);
    }
}

fn sp(s: &Str, fx: usize, k: usize) -> Str {
    let mut res = Str::new();
    for i in 0..fx {
        res.push_back(s[i]);
    }
    res.push_back(s[k]);
    for i in fx..s.len() {
        if i != k { res.push_back(s[i]); }
    }
    res
}

fn lp(s: &Str, fx: usize, keep_first: bool, hs: &mut Res) {
    let kf = 0; //if keep_first { 0usize } else { 1usize };
    for k in (fx + kf)..s.len() {
        let ss = sp(&s, fx, k);
        //count += 1;
        if  ss != *s || keep_first {
            test(&ss, hs);
        }
        lp(&ss, fx + 1, false, hs);
    }
}

fn main() {
    let mut hs = HashSet::new();
    let mut s = nstr(1234567); // manual test of length
    let mut max = 0;
    let m = lp(&s, 0, true, &mut hs);
    for h in hs.iter() {
        if *h > max {
            max = *h;
        }
    }
    println!("{}", max);
}