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 ; } }