16-bit one's complement sum means

1. Add the 16-bit values up. Each time a carry-out (17th bit) is  produced,
 swing that bit around and add it back into the LSb (one's  digit).
 This is somewhat erroneously referred to as "one's complement  addition."
2. Once all the values are added in this manner, invert all the bits  in the result.
 A binary value that has all the bits of another binary  value inverted is called its
 "one's complement," or simply its  "complement."