#include <bits/stdc++.h>
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define REP(i, n)  for(int i=0;i<(n);++i)
#define REPr(i, n) for(int i=(n)-1;i>=0; --i)
#define FORq(i, m, n) for(int i = (m);i <= (n);++i)
#define FORqr(i, m, n) for(int i = (n);i >=(m);--i)
#define PB push_back
#define MP make_pair
#define DEBUG printf("%s\n","debug")
#define fst first
#define snd second
#define SIN(x, S) (S.count(x) != 0)
#define M0(x) memset(x,0,sizeof(x))
#define FILL(x, y) memset(x,y,sizeof(x))
#define MM(x) memset(x,-1,sizeof(x))
#define ALL(x) (x).begin(),(x).end()
#define SCD(n) scanf("%d",&n)
#define SCD2(m,n) scanf("%d%d",&m,&n)
#define SCD3(m,n,k) scanf("%d%d%d",&m,&n,&k)
#define SCLLD(n) scanf("%lld",&n)
#define SCLLD2(m,n) scanf("%lld%lld",&m,&n)
#define SCLLD3(m,n,k) scanf("%lld%lld%lld",&m,&n,&k)
using namespace std;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef long long ll;
typedef long long integer;
///////////////////////////////////////////////
const ll MOD = 1e9+7;
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
template<class T>
inline bool chmin(T &a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
template<class T>
inline bool chmax(T &a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}

template<int MOD> struct Fp {
	long long val;
	constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
		if (val < 0) v += MOD;
	}
	constexpr int getmod() { return MOD; }
	constexpr Fp operator - () const noexcept {
		return val ? MOD - val : 0;
	}
	constexpr Fp operator + (const Fp& r) const noexcept { return Fp(*this) += r; }
	constexpr Fp operator - (const Fp& r) const noexcept { return Fp(*this) -= r; }
	constexpr Fp operator * (const Fp& r) const noexcept { return Fp(*this) *= r; }
	constexpr Fp operator / (const Fp& r) const noexcept { return Fp(*this) /= r; }
	constexpr Fp& operator += (const Fp& r) noexcept {
		val += r.val;
		if (val >= MOD) val -= MOD;
		return *this;
	}
	constexpr Fp& operator -= (const Fp& r) noexcept {
		val -= r.val;
		if (val < 0) val += MOD;
		return *this;
	}
	constexpr Fp& operator *= (const Fp& r) noexcept {
		val = val * r.val % MOD;
		return *this;
	}
	constexpr Fp& operator /= (const Fp& r) noexcept {
		long long a = r.val, b = MOD, u = 1, v = 0;
		while (b) {
			long long t = a / b;
			a -= t * b; swap(a, b);
			u -= t * v; swap(u, v);
		}
		val = val * u % MOD;
		if (val < 0) val += MOD;
		return *this;
	}
	constexpr bool operator == (const Fp& r) const noexcept {
		return this->val == r.val;
	}
	constexpr bool operator != (const Fp& r) const noexcept {
		return this->val != r.val;
	}
	friend constexpr ostream& operator << (ostream &os, const Fp<MOD>& x) noexcept {
		return os << x.val;
	}
	friend constexpr istream& operator >> (istream &is, Fp<MOD>& x) noexcept {
		return is >> x.val;
	}
	friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
		if (n == 0) return 1;
		auto t = modpow(a, n / 2);
		t = t * t;
		if (n & 1) t = t * a;
		return t;
	}
};

///////////////////////////////////////////////
/*(._.)*/
int INF = 1e9;
static vector<PII> E[1022][1022];
int indeg[1022][1022] = {};
int len[1022][1022] = {};
bool visited[1022][1022] = {};
bool cycle = false;
int mcnt = 1000000;
int cnt = 0;
int dfs(int i,int j){
	cnt++;
	if (mcnt < cnt){
		cycle = true;
		return 0;
	}

	visited[i][j] = true;
	if (len[i][j] != 0) {
		visited[i][j] = false;
		return len[i][j];
	}

	int siz = E[i][j].size();
	if (siz == 0){
		visited[i][j] = false;
		return 0;
	}else{
		int ret = 0;
		REP(k,siz){
			if (mcnt < cnt){
				cycle = true;
				return 0;
			}

			if (visited[E[i][j][k].fst][E[i][j][k].snd]) cycle = true;
			ret = max(ret,1 + dfs(E[i][j][k].fst,E[i][j][k].snd));
		}
		visited[i][j] = false;
		return len[i][j] = ret;
	}
}

int main() {
	int N;
	cin >> N;

	vector<vector<int>> A(N + 200);
	REP(i, N) {
		REP(j, N - 1) {
			int a;
			cin >> a;
			A[i + 1].PB(a);
		}
	}

	FORq(i, 1, N) {
		REP(j, N - 2) {
			int nowi = min(i, A[i][j]);
			int nowj = max(i, A[i][j]);
			int nexti = min(i, A[i][j + 1]);
			int nextj = max(i, A[i][j + 1]);
			E[nowi][nowj].PB(MP(nexti, nextj));
			indeg[nexti][nextj]++;
		}
	}
	/*for(int i=1;i<=N;i++){
		for(int j=i + 1;j<=N;j++){
			sort(E[i][j].begin(),E[i][j].end());
			E[i][j].erase(unique(E[i][j].begin(),E[i][j].end()),E[i][j].end());
			for(auto p:E[i][j]){
				indeg[p.first][p.second]++;
			}
		}
	}*/
	//initilize
	FORq(i,1,N){
		FORq(j,1,N){
			if (i >= j) continue;
			if (indeg[i][j] == 0){
				E[0][0].PB(MP(i,j));
			}
		}
	}
	int ans = dfs(0,0);
	if ((cycle) or (ans == 0)) ans = -1;
	cout << ans << endl;
}


Measured memory_bytes: 105216