原式=2+2^20-2^19-2^18.-2^2
=2+(2*2^19-2^19)-2^18.-2^2
=2+2^19-2^18.-2^2
=2+(2*2^18-2^18)-.-2^2
=2+2^18-.-2^2
.
=2+2^2
=4+2
=6