/// Rustで競技プログラミングの入力をスッキリ記述するマクロ
///
/// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

macro_rules! input_inner {
    ($next:expr) => {};
    ($next:expr, ) => {};

    ($next:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($next, $t);
        input_inner!{$next $($r)*}
    };
}

macro_rules! read_value {
    ($next:expr, ( $($t:tt),* )) => {
        ( $(read_value!($next, $t)),* )
    };

    ($next:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    };

    ($next:expr, chars) => {
        read_value!($next, String).chars().collect::<Vec<char>>()
    };

    ($next:expr, usize1) => {
        read_value!($next, usize) - 1
    };

    ($next:expr, $t:ty) => {
        $next().parse::<$t>().expect("Parse error")
    };
}

#[inline]
#[allow(dead_code)]
fn gcd(mut m: u64, mut n: u64) -> u64 {
    if m < n {
        std::mem::swap(&mut m, &mut n);
    }

    loop {
        let k = m % n;
        if k == 0 {
            return n;
        }

        m = n;
        n = k;
    }
}

#[inline]
#[allow(dead_code)]
fn lcm(m: u64, n: u64) -> u64 {
    m / gcd(m, n) * n
}

macro_rules! eprint(
    ($($arg:tt)*) => { {
        use std::io::Write;
        let r = write!(&mut ::std::io::stderr(), $($arg)*);
        r.expect("failed printing to stderr");
    } }
);

macro_rules! eprintln(
    ($($arg:tt)*) => { {
        use std::io::Write;
        let r = writeln!(&mut ::std::io::stderr(), $($arg)*);
        r.expect("failed printing to stderr");
    } }
);

macro_rules! dbg {
    ($val:expr) => {
        // Use of `match` here is intentional because it affects the lifetimes
        // of temporaries - https://stackoverflow.com/a/48732525/1063961
        match $val {
            tmp => {
                eprintln!("[{}:{}] {} = {:#?}",
                          file!(), line!(), stringify!($val), &tmp);
                tmp
            }
        }
    }
}

fn main() {
    use std::collections::BTreeMap;

    input!{
        n: usize,
        s_v: [chars; n],
    }

    let mut s_m_min = BTreeMap::new();
    for &c in s_v[0].iter() {
        *s_m_min.entry(c).or_insert(0u64) += 1;
    }

    for s in s_v.iter().skip(1) {
        // dbg!(&s_m_min);
        // dbg!(s);
        let mut s_m_new: BTreeMap<char, u64> = BTreeMap::new();
        for &c in s.iter() {
            *s_m_new.entry(c).or_insert(0u64) += 1;
        }
        // dbg!(&s_m_new);

        for (c, min_count) in s_m_min.iter_mut() {
            if let Some(new_count) = s_m_new.get(c) {
                *min_count = std::cmp::min(min_count.clone(), *new_count);
            } else {
                *min_count = 0;
            }
        }
    }

    let mut answer = String::new();
    for (c, count) in s_m_min.iter() {
        for _ in 0..*count {
            answer.push(*c);
        }
    }

    println!("{}", answer);
}


Measured memory_bytes: 4476