Prove that, in a group, we do not have to make any distinction between a ‘right identity element’...

Prove that, in a group, we do not have to make any
distinction between a ‘right identity element’ and a ‘left identity element’.
In other words, if i1 and i2 are two particular elements of a group, such that
for all elements a of the group both aΩ i1 D a and i2Ω a D a , then
necessarily i1 D i2. This property also holds for non-Abelian groups!