Plus or minus polyglots!
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.