using System;
using System.IO;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using E = System.Linq.Enumerable;

partial class Solver {
    public int Solve(int n, bool[,] win) {
        var dpL = E.Range(0, n + 1).Select(_ => new BitSet(n + 1)).ToArray(); //new bool[n + 1, n + 1];
        var dpR = E.Range(0, n + 1).Select(_ => new BitSet(n + 1)).ToArray(); //new bool[n + 1, n + 1];
        var winBit = E.Range(0, n).Select(_ => new BitSet(n + 1)).ToArray(); //new bool[n + 1, n + 1];

        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                winBit[i][j] = win[i, j];
            }
        }

        // dpL[i][j] = [i..j] で i が勝ち残る可能性がある
        // dpR[i][j] = [i..j] で j が勝ち残る可能性がある
        // dpL[j][i] = exists k in [i+1,j] where dpL[j][k] && dpR[i+1][k] && win[i][k] == 1
        // dpR[i][j] = exists k in [i,j-1] where dpL[j-1][k] && dpR[i][k] && win[j][k] == 1
        for (int i = 0; i < n; i++) {
            dpL[i][i] = true;
            dpR[i][i] = true;
        }
        for (int d = 1; d <= n - 1; d++) {
            for (int i = 0; i + d < n; i++) {
                var j = i + d;

                var start = i / 64;
                var end = j / 64;
                for (int k = start; k <= end; k++) {
                    if ((winBit[i].LongArray[k] & dpL[j].LongArray[k] & dpR[i + 1].LongArray[k]) != 0) {
                        dpL[j][i] = true;
                        break;
                    }
                }
                for (int k = start; k <= end; k++) {
                    if ((winBit[j].LongArray[k] & dpL[j - 1].LongArray[k] & dpR[i].LongArray[k]) != 0) {
                        dpR[i][j] = true;
                        break;
                    }
                }

                //dpL[j][i] = (winBit[i] & dpL[j] & dpR[i + 1]).Any();
                //dpR[i][j] = (winBit[j] & dpL[j - 1] & dpR[i]).Any();
            }
        }
        int ans = (dpL[n - 1] & dpR[0]).CountBit();
        return ans;
    }
    public void Run() {
        var n = ni();
        var win = new bool[n, n];
        for (int i = 1; i < n; i++) {
            var a = ns();
            for (int j = 0; j < i; j++) {
                win[i, j] = (a[j] == '1');
                win[j, i] = !win[i, j];
            }
        }
        cout.WriteLine(Solve(n, win));
    }
}


public class BitSet {
    public readonly ulong[] _array;

    public BitSet(int size) {
        _array = new ulong[(size - 1) / 64 + 1];
        BitLength = size;
    }

    public int BitLength { get; private set; }

    public int CountBit() {
        int count = 0;
        foreach (var a in _array) {
            count += PopCount(a);
        }
        return count;
    }

    public ulong[] LongArray { get { return _array; } }

    public bool Any() {
        return Array.FindIndex(_array, a => a != 0UL) >= 0;
    }

    public bool All() {
        return CountBit() == BitLength;
    }

    public BitSet Slice(int begin, int end) {
        var bitset = new BitSet(end - begin);
        var jump = begin / 64;
        var move = begin % 64;
        var arrayLength = bitset._array.Length;
        int lastIndex = 0;
        for (int i = 0; i + jump < _array.Length && i < bitset._array.Length; i++) {
            bitset._array[i] = unchecked(
                (_array[i + jump] >> move) |
                ((move > 0 && i + jump + 1 < _array.Length) ? (_array[i + jump + 1] << (64 - move)) : 0)
            );
            lastIndex = i;
        }
        var lastBit = bitset.BitLength & 63;
        if (lastBit > 0) {
            bitset._array[lastIndex] &= (1UL << lastBit) - 1;
        }
        return bitset;
    }

    public static BitSet operator <<(BitSet b, int shift) {
        if (shift == 0) return b;
        var bitset = new BitSet(b.BitLength);
        var jump = shift >> 6;
        var move = shift & 63;
        var arrayLength = bitset._array.Length;
        for (int i = jump; i < arrayLength; i++) {
            bitset._array[i] = unchecked(
                (b._array[i - jump] << move) |
                (move > 0 && i - jump - 1 >= 0 ? (b._array[i - jump - 1] >> (64 - move)) : 0)
            );
        }
        return bitset;
    }

    public static BitSet operator >>(BitSet b, int shift) {
        if (shift == 0) return b;
        var bitset = new BitSet(b.BitLength);
        var jump = shift >> 6;
        var move = shift & 63;
        var arrayLength = bitset._array.Length;
        for (int i = 0; i + jump < arrayLength; i++) {
            bitset._array[i] = unchecked(
                (b._array[i + jump] >> move) |
                (move > 0 && i + jump + 1 < arrayLength ? (b._array[i + jump + 1] << (64 - move)) : 0)
            );
        }
        var lastBit = bitset.BitLength & 63;
        if (lastBit > 0) {
            bitset._array[bitset._array.Length - 1] &= (1UL << lastBit) - 1;
        }
        return bitset;
    }


    public static BitSet operator &(BitSet b1, BitSet b2) {
        var bitset = new BitSet(b1.BitLength);
        for (int i = 0; i < bitset._array.Length; i++) {
            bitset._array[i] = b1._array[i] & b2._array[i];
        }
        return bitset;
    }

    public static BitSet operator |(BitSet b1, BitSet b2) {
        var bitset = new BitSet(b1.BitLength);
        for (int i = 0; i < bitset._array.Length; i++) {
            bitset._array[i] = b1._array[i] | b2._array[i];
        }
        return bitset;
    }

    public static BitSet operator ^(BitSet b1, BitSet b2) {
        var bitset = new BitSet(b1.BitLength);
        for (int i = 0; i < bitset._array.Length; i++) {
            bitset._array[i] = b1._array[i] ^ b2._array[i];
        }
        return bitset;
    }

    private static int PopCount(ulong x) {
        x = (x & 0x5555555555555555UL) + ((x >> 1) & 0x5555555555555555UL);
        x = (x & 0x3333333333333333UL) + ((x >> 2) & 0x3333333333333333UL);
        x = (x + (x >> 4)) & 0x0f0f0f0f0f0f0f0fUL;
        x += x >> 8;
        x += x >> 16;
        x += x >> 32;
        return (int)(x & 0x7FLU);
    }

    public static bool operator ==(BitSet b1, BitSet b2) {
        return b1.Equals(b2);
    }

    public static bool operator !=(BitSet b1, BitSet b2) {
        return !(b1 == b2);
    }

    public override bool Equals(object obj) {
        if (obj is BitSet) {
            return Equals((BitSet)obj);
        }
        return false;
    }

    public bool Equals(BitSet b) {
        if (BitLength != b.BitLength) return false;
        for (int i = 0; i < _array.Length; i++) {
            if (_array[i] != b._array[i]) return false;
        }
        return true;
    }

    public override int GetHashCode() {
        var hash = BitLength.GetHashCode();
        foreach (var a in _array) {
            hash = unchecked(hash * 31) ^ a.GetHashCode();
        }
        return hash;
    }

    public bool this[int index] {
        get {
            return (_array[index >> 6] & (1UL << (index & 63))) != 0UL;
        }
        set {
            if (value) {
                _array[index >> 6] |= 1UL << (index & 63);
            } else {
                _array[index >> 6] &= ~(1UL << (index & 63));
            }
        }
    }

    public override string ToString() {
        var b = new StringBuilder();
        for (int i = BitLength - 1; i >= 0; i--) {
            b.Append(this[i] ? '1' : '0');
        }
        return b.ToString();
    }
}

// PREWRITEN CODE BEGINS FROM HERE
partial class Solver : Scanner {
    public static void Main(string[] args) {
#if LOCAL
        new Solver(Console.In, Console.Out).Run();
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver(Console.In, Console.Out).Run();
        Console.Out.Flush();
#endif
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader) {
        this.cin = reader;
        this.cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer) {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

public class Scanner {
    private readonly TextReader Reader;
    private readonly Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In) {
    }

    public Scanner(TextReader reader) {
        this.Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size) {
        var array = new string[size];
        for (int i = 0; i < size; i++) array[i] = Next();
        return array;
    }
    public int[] NextIntArray(int size) {
        var array = new int[size];
        for (int i = 0; i < size; i++) array[i] = NextInt();
        return array;
    }

    public long[] NextLongArray(int size) {
        var array = new long[size];
        for (int i = 0; i < size; i++) array[i] = NextLong();
        return array;
    }

    public double[] NextDoubleArray(int size) {
        var array = new double[size];
        for (int i = 0; i < size; i++) array[i] = NextDouble();
        return array;
    }

    public string Next() {
        if (TokenQueue.Count == 0) {
            if (!StockTokens()) throw new InvalidOperationException();
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext() {
        if (TokenQueue.Count > 0)
            return true;
        return StockTokens();
    }
    static readonly char[] _separator = new[] { ' ' };
    private bool StockTokens() {
        while (true) {
            var line = Reader.ReadLine();
            if (line == null) return false;
            var tokens = line.Split(_separator, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) continue;
            foreach (var token in tokens)
                TokenQueue.Enqueue(token);
            return true;
        }
    }
}


Measured memory_bytes: 21456