diagonal_difference.rb

# Parse input
n = gets.strip.to_i
a = Array.new(n)
(0...n).each { |a_i|
    a_t = gets.strip
    a[a_i] = a_t.split(' ').map(&:to_i)
    arr = a[a_i]
}

# Initialize variables
a_diagonal = 0
b_diagonal = 0

# Sum diagonals
(0...arr.length).each { |i|
    # Top left to bottom right
    a_diagonal += arr[i][i]
    
    # Top right to bottom left
    b_diagonal += arr[i][arr.length - 1 - i]
}

# Abs of difference of diagonal sums
(a_diagonal - b_diagonal).abs