Cumulative sum over different columns

Dear excel-users,

For my thesis I have to calculate the cumulative returns over multiple days. For now I have the returns already but have to sum them up cumulative per column. See below some returns out my spreadsheet. Only the letters should add cumulative, like a1+a2 etc. Since I have 61 rows, and 717 columns I am looking for a formula I can copy down and to the right. Cause sum($a$1:a1) does not hold for the other columns..

a b c d e
1 0,84% 0,06% -0,08% -0,47% -0,30%
2 3,47% 1,62% -0,32% -0,90% -3,38%
3 -0,41% 0,20% -0,95% 0,15% 1,78%
4 6,37% 0,02% 0,79% 0,23% -0,46%
5 3,32% 0,02% -0,46% 1,05% -0,77%
6 1,49% -1,58% -2,21% 0,76% -3,20%
7 -0,09% -0,46% -0,63% 1,00% -0,33%
8 -3,13% 0,20% -1,71% 0,40% -0,37%

Thank you!