func diagonalSum(mat [][]int) int {
  totalSum := 0
  
  i := 0
  j := len(mat) - 1

  for j >= 0 {
    totalSum += mat[i][i]
    
    if i != j {
      totalSum += mat[i][j]
    }

    i++
    j--
  }

  return totalSum
}