regular expression for the language of even length strings starting with a and ending with b in theory of automata

Regular expression for the language of even length strings starting with a and ending with b

Accepted Strings: 

All strings of the following pattern should be accepted by RE.

ab, aab, abb,abab,……

Reject strings:

All strings of the following pattern should be rejected by RE.

a, b, ba, aa, bb, ba,bba,……

R.E = a (aa+bb+ab+ba)* b