according to the digit 's  position, obtain the last  digit of the applicable  perfect power of 2
if ( $ posFromRight % 4 == 1 ) $ sum = $ sum + 2 ; else if ( $ posFromRight % 4 == 2 ) $ sum = $ sum + 4 ; else if ( $ posFromRight % 4 == 3 ) $ sum = $ sum + 8 ; else if ( $ posFromRight % 4 == 0 ) $ sum = $ sum + 6 ; } }