Competitions

# Checking for an undirected graph

For a given square matrix n x n of zeros and ones determine whether it can be the adjacency matrix of a simple undirected graph. The simple graph does not contain loops and multiple edges.

#### Input

The first line contains the value of n (1n100). Each of the next n lines contains n numbers, describing the adjacency matrix.

#### Output

Print YES, if the graph is simple undirected and NO otherwise.

Time limit 1 second
Memory limit 128 MiB
Input example #1
3
0 1 1
1 0 1
1 1 0

Output example #1
YES

Input example #2
3
0 1 1
1 0 1
0 1 0

Output example #2
NO