You can use the Fixed Format File node to read the file or files. See the Help documentation 'Node help' > 'Input Connectors' > 'Fixed Format File' topic (or online here for the latest version of the product).
A Directory List node can be used to select the required filenames using a pattern, which can then be input to the Fixed Format File node. For example:

As mentioned in the node help, when using this method the format of all the files needs to be exactly the same. If the files do not have the same format the data needs to be input from each file separately and then you can use the Cat node to concatenate the records from each file into a single data set.
When using the Directory List node connected to the input of the Fixed File Format node, the 'FilenameExpr' property is used to specify the input field containing the filenames.

The 'FieldPositions' property is used to specify the expected format of the data in the files. See the node help for details.
Note: there is currently a error in the help documentation relating to the 'FieldPositions' property (which is fixed in the next release)
Instead of: field1,10,5,string:field2:15,3
The specification should be: field1,10,5,string:field2,15,3
Once the data has been imported you can then use a Filter node to output records that match your criteria and then use the Aggregate node to sum the values. Note, in the example screenshot above the field type of the second field (Col_2) was specified as an int (integer) type. By default, all fields are input as string.