Plus or minus polyglots! Your submission must take a list of numbers (in whatever list format your language supports, or using multiple function / command-line parameters) or a string of numbers separated by any character that is not 0123456789. In one language, it must add all of them and output the sum. In another language, it must output them subtracted from each other in order. Example:

12
5
7
2


In one language, it must output 26, and in another it must output -2. Note that all numbers inputted will be positive integers less than 100. There will never be more than 20 numbers provided, so you will never output greater than 2000 or less than -1899. Any questions? Comment below!
Python 2 / 3, 52 bytes

lambda l:sum(l[:1]+[x*int(1-(1/2)*4)for x in l[1:]])


int(1-(1/2)*4) returns 1 in Python 2 because 1/2 evaluates first to 0, and then 0 * 4 = 0.

int(1-(1/2)*4) returns -1 in Python 3 because 1/2 evaluates first to 0.5, and then int(0.5 * 4) = int(2.0) = 2.