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#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr int SIZE = 3005;

void solve() {
    int n,k;
    cin >> n >> k;
    vector<int> w(INT_MAX, SIZE);
    
    for(int i = 1; i <= n; i++) {
        for(int j = 1; j <= i; j++) {
            int t;
            cin >> t;
            w[i] = min(w[i], t);
        }
    }

    int ans = INT_MAX;
    int it = 1;
    while(k > 0) {
        k -= it;
        ans = min(ans, w[it]);
        it++;
    }
    cout << ans << "\n";
}

int main() {
    cin.tie(0);
    ios_base::sync_with_stdio(0);
    solve();
}