1. A charged particle beam writing apparatus comprising: a sum total irradiation time calculation unit configured to obtain a specific value by calculating an integer value by dividing a total irradiation time “n” of charged particle beams of a plurality of times of shooting a same position by a multiplied value, which is obtained by multiplying a multiple writing unit region number “m” being a number of a plurality of multiple writing unit regions for performing multiple writing while shifting a position by a repeating times number “r” being a number of repeating times of performing multiple writing without shifting a position in each of the plurality of multiple writing unit regions, and by multiplying the integer value by the repeating times number “r”, to add the repeating times number “r” to the specific value in a first case that a multiple writing unit region is in the plurality of multiple writing unit regions and is not a specific multiple writing unit region previously set in the plurality of multiple writing unit regions and a multiple writing unit region number of the multiple writing unit region, defined excluding the specific multiple writing unit region, is less than or equal to a value that is obtained by dividing the total irradiation time “n” by the multiplied value, which is calculated by multiplying the multiple writing unit region number “m” by the repeating times number “r”, so as to obtain a first remainder, and further dividing the first remainder by the repeating times number “r”, and to treat an added value of “r” and the specific value, as a sum total irradiation time “a” of a charged particle beam in the multiple writing unit region, in the charged particle beams of the plurality of times of shooting the same position, to treat the specific value as the sum total irradiation time of the charged particle beam in the multiple writing unit region, in the charged particle beams of the plurality of times of shooting the same position, in a second case that the multiple writing unit region is in the plurality of multiple writing unit regions and is not the specific multiple writing unit region previously set in the plurality of multiple writing unit regions and the multiple writing unit region number of the multiple writing unit region, defined excluding the specific multiple writing unit region, is not less than or equal to the value that is obtained by dividing the first remainder by the repeating times number “r”, and to add a second remainder to the specific value in a third case that the multiple writing unit region is the specific multiple writing unit region previously set in the plurality of multiple writing unit regions, where the second remainder is obtained by dividing the total irradiation time “n” by the repeating times number “r”, and to treat an added value of the second remainder and the specific value, as the sum total irradiation time of the charged particle beam in the multiple writing unit region, in the charged particle beams of the plurality of times of shooting the same position; and a writing unit configured to write a pattern on a target object by irradiating the target object with the charged particle beams to be the sum total irradiation time in one case of the first, the second, and the third cases corresponding to each of the plurality of multiple writing unit regions.