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#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);
}