#include<bits/stdc++.h>
#define rep(i, l, r) for(int i = (l); i <= (r); i ++)
#define per(i, r, l) for(int i = (r); i >= (l); i --)
#define trv(i, u, v) for(int i = head[u], v = e[i].to; i; v = e[i = e[i].nxt].to)
#define fi first
#define se second
#define all(s) s.begin(), s.end()
#define sz(s) (int)(s.size())
#define lb(s) ((s) & -(s))
#define pb push_back
#pragma GCC optimize("Ofast,no-stack-protector")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")
using namespace std;

typedef long long ll;
typedef unsigned long long u64;
typedef pair<int, int> P;
mt19937_64 hua(time(0));
template<typename T> inline bool chkmx(T &x, T y) {return x < y ? x = y, 1 : 0;}
template<typename T> inline bool chkmn(T &x, T y) {return y < x ? x = y, 1 : 0;}
template<int T> using A = array<int, T>;

inline int read() {
	int x = 0, f = 1; char c = getchar();
	for(; !isdigit(c); c = getchar()) if(c == '-')  f = 0;
	for(; isdigit(c); c = getchar()) x = x * 10 + c - '0';
	return f ? x : -x;
}
const int maxn = 4000;
const int maxm = 4000000;
const int inf = 1e9;
const int maxv = 64;
const int maxl = 4000 / maxv;
int n, w[maxn + 5][maxn + 5];
struct Bitset {
    u64 a[maxl + 5];
    inline void flip(int p) {
        int u = p / maxv, v = p % maxv;
        a[u] ^= 1ull << v;
    }
    inline void set(int p) {
        int u = p / maxv, v = p % maxv;
        a[u] |= 1ull << v;
    }
    inline bool get(int p) {
        int u = p / maxv, v = p % maxv;
        return a[u] >> v & 1;
	}
    inline Bitset operator & (Bitset b) {
        rep(i, 0, maxl) b.a[i] &= a[i];
        return b; 
    }
    inline Bitset operator ~ () {
    	Bitset b;
        rep(i, 0, maxl) b.a[i] = ~a[i];
        return b; 
    }
    inline int _Find_first(int st = 0) {
		rep(i, st, maxl) if(a[i]) {
			return i * maxv + __builtin_ctzll(a[i]);
		}
		return inf;
	}
}f[maxn + 5], g[maxn + 5];
bitset<maxn + 5> cur[maxn + 5];
P mp[maxm + 5];
void seta(int l, int r) {
	f[l].set(r), g[r].set(l); 
	if(cur[l - 1][r + 1] && !f[l - 1].get(r + 1)) seta(l - 1, r + 1);
	if(r != n) {
		Bitset x = f[r + 1] & ~f[l];
		for(int p = x._Find_first(); p <= n; x.flip(p), p = x._Find_first(p / maxv)) seta(l, p); 
	}
	if(l != 1) {
		Bitset y = g[l - 1] & ~g[r];
		for(int p = y._Find_first(); p <= n; y.flip(p), p = y._Find_first(p / maxv)) seta(p, r); 
	}
}
int main() {
//	freopen("in.txt", "r", stdin);
	n = read();
	rep(i, 1, n) for(int j = i + 1; j <= n; j += 2) w[i][j] = read(), mp[w[i][j]] = {i, j};
	rep(i, 0, n) g[i].set(i + 1), f[i + 1].set(i);
	rep(i, 1,  n * n / 2 / 2) {
		int l = mp[i].fi, r = mp[i].se;
		cur[l][r] = 1;
		if(!f[l].get(r) && f[l + 1].get(r - 1)) seta(l, r);
		if(f[1].get(n)) {
			cout << i << '\n';
			return 0;
		}
	}
	return 0;
}