#define y1 askjdkasldjlkasd
#include <bits/stdc++.h>
#undef y1

using namespace std;

#define pb push_back
#define mp make_pair
#define fi(a, b) for(int i=a; i<=b; i++)
#define fj(a, b) for(int j=a; j<=b; j++)
#define fo(a, b) for(int o=a; o<=b; o++)
#define fdi(a, b) for(int i=a; i>=b; i--)
#define fdj(a, b) for(int j=a; j>=b; j--)
#define fdo(a, b) for(int o=a; o>=b; o--)
#define sz(x) (int)x.size()

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<ll> vll;

#ifdef LOCAL
#define err(...) fprintf(stderr, __VA_ARGS__)
#else
#define err(...) while (0)
#endif

double START_TIME;

void exit() {
#ifdef LOCAL	
	cerr << "TIME: " << setprecision(5) << fixed << (clock() - START_TIME) / CLOCKS_PER_SEC << endl;
#endif	
	exit(0);
}

template<typename A, typename B>
ostream& operator<<(ostream& os, pair<A, B> p) {
	os << "(" << p.first << ", " << p.second << ")";
	return os;
}

template<typename T>
ostream& operator<<(ostream& os, vector<T> v) {
	fi(0, sz(v) - 1) {
		os << v[i] << " ";
	}
	return os;
}

template<typename T>
ostream& operator<<(ostream& os, set<T> t) {
	for (auto z : t) {
		os << z << " ";
	}
	return os;
}

template<typename T1, typename T2>
ostream& operator<<(ostream& os, map<T1, T2> t) {
	cerr << endl;
	for (auto z : t) {
		os << "\t" << z.first << " -> " << z.second << endl;
	}
	return os;
}

#ifdef LOCAL
#define dbg(x) {cerr << __LINE__ << "\t" << #x << ": " << x << endl;}
#define dbg0(x, n) {cerr << __LINE__ << "\t" << #x << ": "; for (int ABC = 0; ABC < n; ABC++) cerr << x[ABC] << ' '; cerr << endl;}
#else
#define dbg(x) while(0){}
#define dbg0(x, n) while(0){}
#endif

#ifdef LOCAL
#define ass(x) if (!(x)) { cerr << __LINE__ << "\tassertion failed: " << #x << endl, abort(); }
#else
#define ass(x) assert(x)
#endif

///////////////////////////////////////////////////

const int MAX = 2e3 + 41;
//const int MAX = 10;

int n;
bitset<MAX> pref[MAX], suf[MAX], win[MAX], can[MAX];
int a[MAX][MAX];

void solve () {
	fi(0, n - 1) {
		fj(0, n - 1) {
			if (a[i][j]) win[i].set(j);
		}
	}
//	fi(0, n - 1) dbg(win[i]);
//	dbg(win[0]);
	fi(0, n - 1) {
		pref[i].set(i);
		suf[i].set(i);
		can[i].set(i);
		fdj(i - 1, 0) {
			if (a[i][j]) {				
				if ( ( (suf[j] << 1) & pref[i]).any() ) {
					pref[i] |= pref[j];
				}
			}
		}
		fdj(i - 1, 0) {
			if ( (pref[i] & (suf[j] << 1)).any() ) {
				can[j].set(i);
			}
		}
		bitset<MAX> sufwin;
		sufwin.reset();
		sufwin.set(i);
		fdj(i - 1, 0) {
			if ( (sufwin & win[j] & can[j]).any() ) {
				sufwin.set(j);
				suf[j].set(i);
			}
		}
/*
		fj(0, i) {
			dbg(pref[j]);
			dbg(suf[j]);
		}
		err("after i = %d\n", i);
*/
	}
	vi answer;
	fi(0, n - 1) {
		if (pref[i].test(0) && suf[i].test(n - 1)) {
			answer.pb(i);
		}
	}
	printf("%d\n", sz(answer));
//	dbg(answer);	
}

int main() {
#ifdef LOCAL
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
	START_TIME = (double)clock();
#endif
	
	scanf("%d", &n);
	fi(1, n - 1) {
		char str[MAX];
		scanf("%s", str);		
		fj(0, i - 1) {			
			a[i][j] = str[j] - '0';
			a[j][i] = 1 - a[i][j];
		}
	}
	solve();

	exit();
	return 0;
}





Measured memory_bytes: 18560