// https://atcoder.jp/contests/m-solutions2019/tasks/m_solutions2019_f
//
#![allow(unused_imports)]
use std::io::*;
use std::io::Write;
use std::fmt::*;
use std::str::*;
use std::cmp::*;
use std::collections::*;

#[allow(unused_macros)]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

#[allow(unused_macros)]
macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

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

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

#[allow(unused_macros)]
macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

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

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

    ($iter:expr, switch) => {
        {
            let ty = read_value!($iter, i32);
            if ty == 1 {
                vec![ty, read_value!($iter, i32), read_value!($iter, i32)]
            } else if ty == 2 {
                vec![ty, read_value!($iter, i32)]
            } else {
                vec![ty, read_value!($iter, i32)]
            }
        }
    };

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

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

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

#[allow(unused_macros)]
macro_rules! dvec {
    ($t:expr ; $len:expr) => {
        vec![$t; $len]
    };

    ($t:expr ; $len:expr, $($rest:expr),*) => {
        vec![dvec!($t; $($rest),*); $len]
    };
}

#[allow(unused_macros)]
macro_rules! ifv {
    ($t:expr, $a:expr, $b: expr) => {
        if $t { $a } else { $b }
    }
}

#[allow(unused_macros)]
macro_rules! fill {
    ($t:expr, $v:expr) => {
        for i in 0..$t.len() {
            $t[i] = $v;
        }
    };
}

#[allow(unused_macros)]
macro_rules! join {
    ($t:expr, $glue:expr) => {
        $t.into_iter().map(|w| w.to_string()).collect::<Vec<_>>().join($glue)
    };
}

#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),*) => {
        println!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

// ===

use std::ops::{Index,IndexMut};
use std::ops::{Shl,Shr,BitOrAssign};


#[derive(Clone, Debug)]
struct BitVector {
    n: usize,
    size: usize,
    data: Vec<u64>
}

impl BitVector {
    fn new(n: usize) -> Self {
        let size = (n+63)/64;
        BitVector {
            n: n,
            size: size,
            data: vec![0; size]
        }
    }

    fn get(&self, i: usize) -> bool {
        self.data[i/64]>>(i%64) & 1 != 0
    }

    fn set(&mut self, i: usize) {
        self.data[i/64] |= 1<<(i%64);
    }
}

impl<'a> Shl<usize> for &'a BitVector {
    type Output = BitVector;

    fn shl(self, amount: usize) -> Self::Output {
        let mut new_vector = BitVector::new(self.n);

        let diff = amount / 64;
        let diff_mod = amount % 64;
        if diff_mod == 0 {
            for i in 0..self.size-diff {
                new_vector.data[i+diff] = self.data[i];
            }
        } else {
            let lower_mask = (1<<(64-diff_mod))-1;
            let upper_mask = ((1<<diff_mod)-1)<<(64-diff_mod);
            for i in 0..self.size-diff {
                new_vector.data[i+diff] |= (self.data[i] & lower_mask)<<diff_mod;
                if i+diff+1 < self.size {
                    new_vector.data[i+diff+1] |= (self.data[i] & upper_mask)>>(64-diff_mod);
                }
            }
        }
        new_vector
    }
}

impl BitOrAssign for BitVector {
    fn bitor_assign(&mut self, rhs: Self) {
        for i in 0..self.size {
            self.data[i] |= rhs.data[i];
        }
    }
}

// ====

fn check(from: usize, to: usize, win_vector: &Vec<BitVector>, dp_left: &Vec<BitVector>, dp_left0: &Vec<BitVector>, dp_left_row: &Vec<BitVector>, dp_right: &Vec<BitVector>) -> bool {
    let v = win_vector[0].data.len();
    for i in 0..v {
        if dp_left0[from].data[i] & win_vector[from].data[i] & dp_left_row[to].data[i] >= 1 {
            // debug!("found-l", from, to);
            return true;
        }
        if dp_left0[from].data[i] & dp_right[to].data[i] >= 1 && win_vector[from].get(to) {
            // debug!("found-r", from, to);
            return true;
        }
        if dp_left[from].data[i] & dp_left_row[to].data[i] >= 1 {
            return true;
        }
    }
    false
}

fn main() {
    input! {
        n: usize,
        a: [chars; n-1]
    };

    let mut table = dvec!(false; n, n);
    for i in 0..n {
        for j in 0..i {
            table[i][j] = ifv!(a[i-1][j] == '0', false, true);
            table[j][i] = table[i][j] ^ true;
        }
        table[i][i] = true;
    }

    let mut win_vector = vec![BitVector::new(n); n];
    for i in 0..n {
        for j in 0..n {
            if table[i][j] {
                win_vector[i].set(j);
            }
        }
    }

    let mut dp_left = vec![BitVector::new(n); n];
    let mut dp_right = vec![BitVector::new(n); n];
    let mut dp_left_row = vec![BitVector::new(n); n];
    let mut dp_right_row = vec![BitVector::new(n); n];
    let mut dp_left0 = vec![BitVector::new(n); n];
    let mut dp_right0 = vec![BitVector::new(n); n];
    for i in 0..n {
        dp_left[i].set(i);
        dp_right[i].set(i);
        if i >= 1 {
            dp_left0[i].set(i-1);
        }
        if i+1 < n {
            dp_right0[i].set(i+1);
        }
        dp_left_row[i].set(i);
        dp_right_row[i].set(i);
    }

    for d in 1..n {
        for i in 0..n {
            if i >= d {
                if check(i, i-d, &win_vector, &dp_left, &dp_left0, &dp_left_row, &dp_right) {
                    dp_left[i].set(i-d);
                    if i-d >= 1 {
                        dp_left0[i].set(i-d-1);
                    }
                    dp_left_row[i-d].set(i);
                }
            }
            if i+d < n {
                if check(i, i+d, &win_vector, &dp_right, &dp_right0, &dp_right_row, &dp_left) {
                    dp_right[i].set(i+d);
                    if i+d+1 < n {
                        dp_right0[i].set(i+d+1);
                    }
                    dp_right_row[i+d].set(i);
                }
            }
        }
    }

    let mut count = 0;
    // for i in 0..n {
    //     for j in 0..n {
    //         let b = if i < j {
    //             dp_right[i].get(j)
    //         } else {
    //             dp_left[i].get(j)
    //         };
    //         debug!(i, j, b);
    //     }
    // }

    for i in 0..n {
        if dp_left[i].get(0) && dp_right[i].get(n-1) {
            count += 1;
        }
    }
    println!("{}", count);

}


Measured memory_bytes: 22780