Concatenate two files

There are two files with text in them. The first one has all the lines enumerated like in the example below:
1. Individuals and Interactions Over Processes and Tools
2. Working Software Over Comprehensive Documentation
3. Customer Collaboration Over Contract Negotiation
4. Responding to Change Over Following a Plan
The second one just has lines one after another. You are asked to merge the second one into the first one by continuing the enumeration and appending the lines from the second file.
The input contains two lines - the name of the first file and the name of the second one.
The program should merge the files.
Input
doc.txt
second.txt

doc.txt
1. Individuals and Interactions Over Processes and Tools
2. Working Software Over Comprehensive Documentation
3. Customer Collaboration Over Contract Negotiation
4. Responding to Change Over Following a Plan

second.txt
Customer satisfaction through early and continuous software delivery
Accommodate changing requirements throughout the development process
Frequent delivery of working software
Collaboration between the business stakeholders and developers throughout the project
Support, trust, and motivate the people involved
Enable face-to-face interactions
Working software is the primary measure of progress
Agile processes to support a consistent development pace
Attention to technical detail and design enhances agility
Simplicity
Self-organizing teams encourage great architectures, requirements, and designs
Regular reflections on how to become more effective

Resulting doc.txt
1. Individuals and Interactions Over Processes and Tools
2. Working Software Over Comprehensive Documentation
3. Customer Collaboration Over Contract Negotiation
4. Responding to Change Over Following a Plan
5. Customer satisfaction through early and continuous software delivery
6. Accommodate changing requirements throughout the development process
7. Frequent delivery of working software
8. Collaboration between the business stakeholders and developers throughout the project
9. Support, trust, and motivate the people involved
10. Enable face-to-face interactions
11. Working software is the primary measure of progress
12. Agile processes to support a consistent development pace
13. Attention to technical detail and design enhances agility
14. Simplicity
15. Self-organizing teams encourage great architectures, requirements, and designs
16. Regular reflections on how to become more effective
Note that the filenames are provided in the input and are not always set to doc.txt or second.txt.
 

Constraints

Time limit: 2 seconds

Memory limit: 512 MB

Output limit: 1 MB

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