How do you get the numerical value from a string of digits?

I need to add certain parts of the numerical string.

for example like.

036000291453

I want to add the numbers in the odd numbered position so like

0+6+0+2+1+5 and have that equal 14.

I tried the charAt(0)+charAt(2) etc, but it returns the digit at those characters instead of adding them. Thanks for your help.