Sunday Times Teaser 2756 – Terrible Teens

by Victor Bryant

I have given letters numerical values so that words can be valued by adding the values of their letters.

These letter values may be positive or negative and some letters may have the same values.

It turns out that NONE has the value 0, while the words ONE to ELEVEN each have the value that they define (ONE = 1, TWO = 2, etc.).

However only two of the words TWELVE to NINETEEN have the values that they define.

Which two?