1+1 = 1 is love

Nine consecutive integers are placed in the nine squares of a 3x3 grid. If every column, every row, and every diagonal adds up to the same number n and the middle number is 8 what is n?

If

a+b=8, a+c=13, b+d=8, c-d=6

Then

a, b, c, d = ??

Can you arrange the numbers from 1 to 9 on a tic tac toe board in a manner that the numbers in each row, column and diagonal adds up to 15. Remember that you have to use all the 9 numbers and thus you cannot repeat.