#include <bits/stdc++.h>
#define ADD(a, b) a = (a + ll(b)) % mod
#define MUL(a, b) a = (a * ll(b)) % mod
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define rep(i, a, b) for(int i = int(a); i < int(b); i++)
#define rer(i, a, b) for(int i = int(a) - 1; i >= int(b); i--)
#define all(a) (a).begin(), (a).end()
#define sz(v) (int)(v).size()
#define pb push_back
#define sec second
#define fst first
#define debug(fmt, ...) Debug(__LINE__, ":", fmt, ##__VA_ARGS__)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<int, pi> ppi;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vl> mat;
typedef complex<double> comp;
void Debug() {cerr << '\n'; }
template<class FIRST, class... REST>void Debug(FIRST arg, REST... rest){
	cerr<<arg<<" ";Debug(rest...);}
template<class T>ostream& operator<<(ostream& out,const vector<T>& v) {
	out<<"[";if(!v.empty()){rep(i,0,sz(v)-1)out<<v[i]<<", ";out<<v.back();}out<<"]";return out;}
template<class S, class T>ostream& operator<<(ostream& out,const pair<S, T>& v){
	out<<"("<<v.first<<", "<<v.second<<")";return out;}
const int MAX_N = 500010;
const int MAX_V = 100010;
const double eps = 1e-6;
const ll mod = 1000000007;
const int inf = 1 << 30;
const ll linf = 1LL << 60;
const double PI = 3.14159265358979323846;
mt19937 rng; //use it by rng() % mod, shuffle(all(vec), rng)
///////////////////////////////////////////////////////////////////////////////////////////////////

int N;
bitset<2048> A[2010];

bitset<2048> L[2010]; //left i is reachable from where
bitset<2048> R[2010]; //right i
bitset<2048> W[2010]; //i can win where
int LW[2010][2010]; //left i can win
int RW[2010][2010];

bitset<2048> range(bitset<2048> bit, int l, int r) {
	bit >> l;
	bit << l;
	bit << (2048 - r - 1);
	bit >> (2048 - r - 1);
	return bit;
}

void solve() {
	cin >> N;
	rep(i, 1, N) {
		string str; cin >> str;
		rep(j, 0, i) {
			A[i][j] = (str[j] == '1');
			A[j][i] = A[i][j] ^ 1;
		}
	}
	vector<int> vec;
	rep(i, 0, N - 1) vec.pb(i);
	// set<int> S;
	// do {
	// 	vector<int> tmp;
	// 	rep(i, 0, N) tmp.pb(i);
	// 	vector<vi> omo;
	// 	omo.pb(tmp);
	// 	rep(i, 0, N - 1) {
	// 		int a = tmp[vec[i]];
	// 		int b = tmp[vec[i] + 1];
	// 		if(A[a][b] == 0) swap(a, b);
	// 		rep(j, 0, N) {
	// 			if(tmp[j] == b) tmp[j] = a;
	// 		}
	// 		omo.pb(tmp);
	// 	}
	// 	if(tmp.front() == 0) {
	// 		debug(vec);
	// 		debug(omo);
	// 	}
	// 	S.insert(tmp.front());
	// } while(next_permutation(all(vec)));
	// cout << sz(S) << "\n";
	// debug(vi(all(S)));

	rep(i, 0, N) {
		LW[i][i] = 1;
		RW[i][i] = 1;
		L[i][i] = 1;
		R[i][i] = 1;
	}
	bitset<2048> mask;
	rep(i, 0, N) mask[i] = 1;

	rep(l, 2, N + 1) {
		rep(i, 0, N - l + 1) {
			int j = i + l - 1;

			// LW
			auto w = range(A[i], i, j);
			if((w & L[i + 1] & R[j]).any()) LW[i][j] = 1;

			// RW
			w = range(A[j], i, j);
			if((w & L[i] & R[j - 1]).any()) RW[i][j] = 1;
			
			// debug(i, j, LW[i][j], RW[i][j]);
		}
		rep(i, 0, N - l + 1) {
			int j = i + l - 1;
			if(RW[i][j]) L[i][j] = 1;
			if(LW[i][j]) R[j][i] = 1;
		}
	}
	int res = 0;
	rep(i, 0, N) {
		if(RW[0][i] & LW[i][N - 1]) {
			// debug(i);
			res++;
		}
	}
	cout << res << "\n";
}

uint32_t rd() {
	uint32_t res;
#ifdef __MINGW32__
	asm volatile("rdrand %0" :"=a"(res) ::"cc");
#else
	res = std::random_device()();
#endif
	return res;
}

int main() {
#ifndef LOCAL
	ios::sync_with_stdio(false);
    cin.tie(0);
#endif
    cout << fixed;
	cout.precision(20);
    cerr << fixed;
	cerr.precision(6);
	rng.seed(rd());
#ifdef LOCAL
	//freopen("in.txt", "wt", stdout); //for tester
    freopen("in.txt", "rt", stdin);
#endif	
	solve();
    cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
	return 0;
}



Measured memory_bytes: 33920