P3073 [USACO13FEB] Tractor S

One of Farmer John's fields is particularly hilly, and he wants to purchase a new tractor to drive around on it. The field is described by an N x N grid of non-negative integer elevations (1

\* Line 1: The value of N. \* Lines 2..1+N: Each line contains N space-separated non-negative integers (each at most 1 million) specifying a row of FJ's field.

\* Line 1: The minimum cost of a tractor that is capable of driving around at least half of FJ's field.