#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define L long long
#define MP make_pair
#define REP(i, n) for(int i = 0; i < n; ++i)
#define REPR(i, n) for(int i = n - 1; i >= 0; --i)
#define FOR(i, a, b) for(int i = a; i < b; ++i)
#define FORR(i, a, b) for(int i = b - 1; i >= a; --i)
#define EB emplace_back
#define ST first
#define ND second
#define S size()
#define RS resize
using VI = vector<int>;
using VVI = vector<VI>;
using PI = pair<int, int>;
using VPI = vector<PI>;
using VVPI = vector<VPI>;
using VL = vector<L>;
using VVL = vector<VL>;
using VB = vector<bool>;
using VC = vector<char>;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, k;
cin >> n >> k;
VVI w(n, VI(n));
REP(i, n)
REP(j, i + 1)
cin >> w[i][j];
VVI dp(n, VI(n));
REP(i, n)
dp[i][0] = dp[i][i] = i + 1;
FOR(i, 2, n)
FOR(j, 1, i)
dp[i][j] = dp[i - 1][j - 1] + dp[i - 1][j] - dp[i - 2][j - 1] + 1;
int wyn = 2020;
REP(i, n)
REP(j, i + 1)
if (dp[i][j] <= k)
wyn = min(wyn, w[i][j]);
cout << wyn;
}