For thinking about nested loops, it might help to label the circles by column and row.
EDIT (September 29, 2022):
Within each pair of numbers in all of the circles in the sketch, the first number represents the value of the control variable of the outer loop and the second represents the value of the control variable of the inner loop. These are the column and row numbers.
Take a look at the pairs of numbers in the circles in the diagonal. Is there a consistency regarding the sum of the two numbers in the pairs?
Also note the sums of the pairs of numbers below the diagonal. For each one, how does it relate to the sum of the two numbers in the circle in the diagonal above?
Do these observations suggest anything regarding what would be a good header for the inner loop? There are several possibilities that would work.