#include <bits/stdc++.h>
#define PB push_back
#define ST first
#define ND second
#define _ ios_base::sync_with_stdio(0); cin.tie(0);
//mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());

using namespace std;

using ll = long long;
using pi = pair<int,int>;
using vi = vector<int>;

const int nax = 2000+10,INF=1e9;
int n,k;
int ans=INF;

int main() {_
	cin>>n>>k;
	for(int i=1; i<=n; i++) {
		for(int j=1; j<=i; j++) {
			int x;
			cin>>x;
			int L = (j-1)*j/2, R = (i-j)*(i-j+1)/2, ALL = i*(i+1)/2;
			if(ALL-L-R<=k) ans=min(ans,x);
		}
	}
	cout<<ans;
	
}