Kata

German mathematician Christian Goldbach (1690-1764) conjectured that every even number greater than 2 can be represented by the sum of two prime numbers. For example, 10 can be represented as 3+7 or 5+5.

Your job is to make the function return a list containing all unique possible representations of n in an increasing order if n is an even integer; if n is odd, return an empty list. Hence, the first addend must always be less than or equal to the second to avoid duplicates.

Constraints : 2 < n < 32000 and n is even

Examples

26  -->  ['3+23', '7+19', '13+13']

100 -->  ['3+97', '11+89', '17+83', '29+71', '41+59', '47+53']

7   -->  []