#Kona 16 chars

{((+/x)%2)_in x}

Takes a vector from the stack, sums them, divides by 2 and determines if it's in the vector. Returns 1 as truthy and 0 as falsey.

Called via

> {((+/x)%2)_in x} [(2;3;5)]
1
> {((+/x)%2)_in x} [(2;3;4)]
0

Kona 16 chars

{((+/x)%2)_in x}

Takes a vector from the stack, sums them, divides by 2 and determines if it's in the vector. Returns 1 as truthy and 0 as falsey.

Called via

> {((+/x)%2)_in x} [(2;3;5)]
1
> {((+/x)%2)_in x} [(2;3;4)]
0