#include <bits/stdc++.h>
namespace // my guiding star
{
#define filein(x) freopen(x".in", "r", stdin);
#define fileout(x) freopen(x, "w", stdout);
#define fileerr(x) freopen(x, "w", stderr);
#define file(x) freopen(x".in", "r", stdin), freopen(x".out", "w", stdout);
#define files(x) freopen(x".in", "r", stdin), freopen(x".ans", "w", stdout);
using namespace std;
#define cT const T&
template<typename T>
inline T chkmin(T& x, cT y){if (x > y) x = y; return x;}
template<typename T>
inline T chkmax(T& x, cT y){if (x < y) x = y; return x;}
template <typename T>
inline bool inrange(cT x, cT l, cT r){return (l <= x) && (x <= r);}
template <typename T>
inline bool inrange(cT l, cT r, cT L, cT R){return (L <= l) && (r <= R);}
#undef cT
typedef long long ll;
typedef unsigned long long ull;
typedef double db;
typedef long double ldb;
typedef unsigned u32;
template <typename T>
using pr = pair<T, T>;
typedef pr<int> pii;
typedef pr<ll> pll;
typedef pr<db> pdd; 
typedef complex<double> cp;
typedef vector<int> vi;
inline void YN(bool x){puts(x ? "Yes" : "No");}
}
const int N = 4444;
int n;
pii a[N];
bitset<N> f[N], g[N], fp[N], gp[N];
bool v[N][N];
void solve(int x, int y)
{
	if (f[x][y]) return ;
	f[x][y] = g[y][x] = 1; fp[x][y] = gp[y][x] = 0;
	if (v[x - 1][y + 1]) solve(x - 1, y + 1);
	auto B = fp[x] & f[y + 1];
	for (int i=B._Find_next(y); i!=B.size(); i=B._Find_next(i)) solve(x, i);
	B = gp[y] & g[x - 1];
	for (int i=B._Find_first(); i<x; i=B._Find_next(i)) solve(i, y);
}
int main()
{
	scanf("%d", &n);
	for (int i=1; i<=n; i++){fp[i].set(); gp[i].set();}
	for (int i=1, x; i<n; i++)
		for (int j=i+1; j<=n; j+=2){scanf("%d", &x); a[x] = {i, j};}
	for (int i=1; i<=n; i++) f[i][i - 1] = g[i - 1][i] = 1;
	for (int i=1; i<=n*n/4; i++)
	{
		int x, y; tie(x, y) = a[i]; v[x][y] = true;
		if (f[x + 1][y - 1]) solve(x, y);
		if (f[1][n]) return printf("%d\n", i), 0;
	}
	puts("-1");
	return 0;
}