Problem:
Score : 1000 points

Problem Statement

We will host a rock-paper-scissors tournament with N people. The participants are called Person 1, Person 2, \ldots, Person N.
For any two participants, the result of the match between them is determined in advance.
This information is represented by positive integers A_{i,j} ( 1 \leq j < i \leq N ) as follows:

If A_{i,j} = 0, Person j defeats Person i.

If A_{i,j} = 1, Person i defeats Person j.

The tournament proceeds as follows:

We will arrange the N participants in a row, in the order Person 1, Person 2, \ldots, Person N from left to right.

We will randomly choose two consecutive persons in the row. They will play a match against each other, and we will remove the loser from the row.
We will repeat this process N-1 times, and the last person remaining will be declared the champion.

Find the number of persons with the possibility of becoming the champion.

Constraints

1 \leq N \leq 2000

A_{i,j} is 0 or 1.

Input

Input is given from Standard Input in the following format:

N
A_{2,1}
A_{3,1}A_{3,2}
:
A_{N,1}\ldotsA_{N,N-1}

Output

Print the number of persons with the possibility of becoming the champion.

Sample Input 1

3
0
10

Sample Output 1

2

Person 1 defeats Person 2, Person 2 defeats Person 3 and Person 3 defeats Person 1.
If Person 1 and Person 2 play the first match, Person 3 will become the champion.
If Person 2 and Person 3 play the first match, Person 1 will become the champion.

Sample Input 2

6
0
11
111
1111
11001

Sample Output 2

3

Solution in Rust (1.15.1) (judge verdict: Accepted)

#[allow(unused_imports)]
use std::cmp::*;
#[allow(unused_imports)]
use std::collections::*;
use std::io::{Write, BufWriter};
// https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    ($($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, [ $t:tt ]) => {{
        let len = read_value!($next, usize);
        (0..len).map(|_| read_value!($next, $t)).collect::<Vec<_>>()
    }};

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

// Verified by https://atcoder.jp/contests/arc084/submissions/3935443
#[derive(Clone)]
struct BitSet {
    size: usize,
    buf: Vec<usize>,
}

impl BitSet {
    // size should be a multiple of bit-size of usize.
    fn new(size: usize) -> Self {
        let w = 8 * std::mem::size_of::<usize>();
        assert_eq!(size & (w - 1), 0);
        let count = size / w;
        BitSet {
            size: size,
            buf: vec![0; count],
        }
    }
    #[allow(unused)]
    fn set(&mut self, idx: usize, val: bool) {
        debug_assert!(idx < self.size);
        let w = 8 * std::mem::size_of::<usize>();
        let idx0 = idx / w;
        let idx1 = idx & (w - 1);
        if val {
            self.buf[idx0] |= 1 << idx1;
        } else {
            self.buf[idx0] &= !(1 << idx1);
        }
    }
    #[allow(unused)]
    fn get(&self, idx: usize) -> bool {
        let w = 8 * std::mem::size_of::<usize>();
        debug_assert!(idx < self.size);
        let idx0 = idx / w;
        let idx1 = idx & (w - 1);
        (self.buf[idx0] >> idx1 & 1) == 1
    }
    #[allow(unused)]
    fn shl(&self, val: usize) -> Self {
        if val >= self.size { return Self::new(self.size); }
        let w = 8 * std::mem::size_of::<usize>();
        let count = self.size / w;
        let sh0 = val / w;
        let sh1 = val & (w - 1);
        let mut ans = Self::new(self.size);
        if sh1 == 0 {
            for i in 0 .. count - sh0 {
                ans.buf[i + sh0] = self.buf[i];
            }
        } else {
            ans.buf[sh0] = self.buf[0] << sh1;
            for i in 1 .. count - sh0 {
                ans.buf[i + sh0] = self.buf[i] << sh1
                    | self.buf[i - 1] >> (w - sh1);
            }
        }
        ans
    }
    // Not verified
    #[allow(unused)]
    fn shr(&self, val: usize) -> Self {
        if val >= self.size { return Self::new(self.size); }
        let w = 8 * std::mem::size_of::<usize>();
        let count = self.size / w;
        let sh0 = val / w;
        let sh1 = val & (w - 1);
        let mut ans = Self::new(self.size);
        if sh1 == 0 {
            for i in 0 .. count - sh0 {
                ans.buf[i] = self.buf[i + sh0];
            }
        } else {
            for i in 0 .. count - sh0 - 1 {
                ans.buf[i] = self.buf[i + sh0] >> sh1
                    | self.buf[i + sh0 + 1] << (w - sh1);
            }
            ans.buf[self.size - sh0 - 1] = self.buf[self.size - 1] >> sh1;
        }
        ans
    }
    #[allow(unused)]
    fn msb(&self) -> Option<usize> {
        let w = 8 * std::mem::size_of::<usize>();
        let count = self.size / w;
        for i in (0 .. count).rev() {
            let v = self.buf[i];
            if v != 0 {
                return Some(w * i + w - 1 - v.leading_zeros() as usize);
            }
        }
        None
    }
}

trait BitSetAdapter: Sized {
    fn word_size(&self) -> usize;
    fn at(&self, word_pos: usize) -> usize;
    // short-circuit operations
    fn any(&self) -> bool {
        let sz = self.word_size();
        for i in 0..sz {
            if self.at(i) != 0 {
                return true;
            }
        }
        false
    }
    fn all(&self) -> bool {
        let sz = self.word_size();
        for i in 0..sz {
            if self.at(i) != 1usize.wrapping_neg() {
                return false;
            }
        }
        true
    }
    fn and<U: BitSetAdapter>(self, other: U) -> AndAdapter<Self, U> {
        AndAdapter {
            t: self,
            u: other,
        }
    }
}

struct AndAdapter<T, U> {
    t: T,
    u: U,
}

impl<T: BitSetAdapter, U: BitSetAdapter> BitSetAdapter for AndAdapter<T, U> {
    fn word_size(&self) -> usize {
        self.t.word_size()
    }
    fn at(&self, word_pos: usize) -> usize {
        self.t.at(word_pos) & self.u.at(word_pos)
    }
}

/*
impl BitSetAdapter for BitSet {
    fn word_size(&self) -> usize {
        let w = 8 * std::mem::size_of::<usize>();
        assert_eq!(self.size & (w - 1), 0);
        self.size / w
    }
    fn at(&self, word_pos: usize) -> usize {
        self.buf[word_pos]
    }
}*/

impl<'a> BitSetAdapter for &'a BitSet {
    fn word_size(&self) -> usize {
        let w = 8 * std::mem::size_of::<usize>();
        assert_eq!(self.size & (w - 1), 0);
        self.size / w
    }
    fn at(&self, word_pos: usize) -> usize {
        self.buf[word_pos]
    }
}

fn solve() {
    let out = std::io::stdout();
    let mut out = BufWriter::new(out.lock());
    macro_rules! puts {
        ($($format:tt)*) => (write!(out,$($format)*).unwrap());
    }
    input! {
        n: usize,
        aa: [chars; n - 1],
    }
    let mut a = vec![BitSet::new(2048); n];
    let mut dp = vec![BitSet::new(2048); n];
    for i in 1..n {
        for j in 0..i {
            a[i].set(j, aa[i - 1][j] == '1');
            a[j].set(i, aa[i - 1][j] == '0');
        }
    }
    for i in 0..n {
        dp[i].set(i, true);
    }
    for s in 1..n {
        for i in 0..n - s {
            let j = i + s;
            let res = dp[i].and(&dp[j - 1]).and(&a[j]).any();
            dp[i].set(j, res);
            let res = dp[i + 1].and(&dp[j]).and(&a[i]).any();
            dp[j].set(i, res);
        }
    }
    let ans = (0..n)
        .filter(|&i|
                dp[0].get(i) && dp[n - 1].get(i))
        .count();
    puts!("{}\n", ans);
}

fn main() {
    // In order to avoid potential stack overflow, spawn a new thread.
    let stack_size = 104_857_600; // 100 MB
    let thd = std::thread::Builder::new().stack_size(stack_size);
    thd.spawn(|| solve()).unwrap().join().unwrap();
}