#include <algorithm>
#include <stdio.h>
#include <limits.h>
using namespace std;
int MAX_VAL = 20;
int main(){
long long int height;
long long int needed;
long long int year;
long long int lowest_year = LLONG_MAX;
scanf("%lld %lld", &height, &needed);
for (int idx_col = 0; idx_col < height; ++idx_col) {
for (int idx_row = 0; idx_row <= idx_col; ++idx_row) {
scanf("%lld", &year);
if ((idx_col - idx_row + 1) * (idx_row + 1) <= needed) {
lowest_year = min(lowest_year, year);
}
}
}
printf("%lld\n", lowest_year);
return 0;
}