#include <bits/stdc++.h>
typedef unsigned long long ull;
using namespace std;
namespace io{
	const int SIZE = (1 << 24) + 1;
	char ibuf[SIZE], *iS, *iT;
	char obuf[SIZE], *oS = obuf, *oT = oS + SIZE - 1, c, qu[55];
	int f, qr;
	
	#define gc() (iS == iT ? (iT = (iS = ibuf) + fread (ibuf, 1, SIZE, stdin), (iS == iT ? EOF : *iS ++)) : *iS ++)
	
	inline void flush () {
		fwrite (obuf, 1, oS - obuf, stdout);
		oS = obuf;
	}
	
	inline void putc (char x) {
		*oS ++ = x;
		if (oS == oT) flush ();
	}
	
	template <class I>
	inline void read (I &x) {
		for (f = 1, c = gc(); c < '0' || c > '9'; c = gc()) if (c == '-') f = -1;
		for (x = 0; c <= '9' && c >= '0'; c = gc()) x = x * 10 + (c & 15); 
		x = f == -1 ? -x : x;
	}
	
	template <class I>
	inline void print (I x) {
		if (!x) putc ('0'); if (x < 0) putc ('-'), x = -x;
		while (x) qu[++ qr] = x % 10 + '0',  x /= 10;
		while (qr) putc (qu[qr --]);
	}
	
	struct Flusher_ {~Flusher_(){flush();}}io_flusher_;
}
using io :: read;
using io :: putc;
using io :: print;
struct op{
	int l,r;
}p[4000010];
int n,m;
struct bits{
	ull b[63];
	void set(int i){
		b[i>>6]|=1ll<<(i&63);
	}
	bool test(int i){
		return (b[i>>6]>>(i&63))&1;
	}
	bits operator &(bits a){
		bits res;
		for(int i=0;i<63;i++) res.b[i]=b[i]&a.b[i];
		return res;
	}
	bits operator ^(bits a){
		bits res;
		for(int i=0;i<63;i++) res.b[i]=b[i]^a.b[i];
		return res;
	}
	int _Find_first(){
		for(int i=0;i<63;i++) if(b[i]) return __builtin_ctzll(b[i])+i*64;
		return -1;
	}
}l[4001],r[4001],can[8001];
int u;
void in(int L,int R){
	l[L].set(R),r[R].set(L);
	while((u=(l[R+1]^(l[L]&l[R+1]))._Find_first())>=0){
		in(L,u);
	}
	while((u=(r[L-1]^(r[R]&r[L-1]))._Find_first())>=0){
		in(u,R);
	} 
	if(can[L+R].test(L-1)&&!l[L-1].test(R+1)) in(L-1,R+1);
}
int main(){
	read(n);
	for(int i=1,w;i<=n;i++){
		for(int j=i+1;j<=n;j+=2) read(w),p[w]={i,j},m++;
	}
	for(int i=1;i<n;i++) in(i+1,i);
	for(int i=1;i<=m;i++){
		int L=p[i].l,R=p[i].r;
		can[L+R].set(L);
		if(l[L+1].test(R-1)) in(L,R);
		if(l[1].test(n)) print(i),putc('\n'),exit(0);
	}
}