Bromine atom is available in the form of two isotopes 79/35 br(49.7%)and 81/35 br(50.3%)calculate atomic m?

the que is : if bromine atom is avilable in the form of say two isotopes 79/35 Br(49.7%)and 81/35 Br(50.3%) , calculate the average atomic m of bromine atom ?? please answer in brief step by step...........