#include <algorithm>
#include <cstdio>
#include <vector>
using namespace std;
int main()
{
int n, k;
scanf("%d%d", &n, &k);
vector<vector<int>> d(n);
vector<vector<int>> m(n);
for(int i = 0; i < n; ++i)
{
d[i].reserve(i+1);
m[i].resize(i+1);
for(int j = 0; j <= i; ++j)
{
int a;
scanf("%d", &a);
d[i].push_back(a);
}
}
for(int i = 0; i < n; ++i)
{
m[i].front() = i + 1;
m[i].back() = i + 1;
}
for(int i = 2; i < n; ++i)
{
for(int j = 1; j < (m[i].size()-1); ++j)
{
m[i][j] = i + (static_cast<int>(m[i].size()) - i) + m[i-2][j-1];
}
}
int res = d[0][0];
for(int i = 0; i < n; ++i)
{
for(int j = 0; j < d[i].size(); ++j)
{
if(m[i][j] <= k)
res = min(res, d[i][j]);
}
}
printf("%d\n", res);
}