How to find the index of an element through a string?

Question

Mathematics

There is such a matrix. When you click on an element with an index, you need to get its value.

For example, when you click on 1 index in 1 row, you should get a total of 4.
Row + 1 = 4.
Tell me what the formula should be.

Answer 1

Something like
where m is the number of columns in the matrix

Answer 2

You yourself can easily solve this problem if you formulate the question correctly.
Element or index?
For what unit? You have five of them. Perhaps if you put commas in this sentence, it will become a little clearer.

Row + 1 = 4.
Tell me what the formula should be.

Isn't that a formula?
PS sum = i*N + j, where i is the row number, j is the column number, N is the number of columns.