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Numbered regions (726)

Instruction:
There are 20 regions outlined in the map. Assign one number from 1 to 20 to each region so that each number is used exactly once.
The numbers in the shaded cells indicate the sum of the numbers in regions sharing at least one side with this cell (each region is counted just once in the sum).

Reference: WPC 2018 (120)

Related puzzles:
Sum Divide