Software :: Join 2 Text Files Based On First Number Present In Every Line Of The 2 Text Files?

I have 2 text files : file1.txt and file2.txt

cat file1.txt

15 this is a sentence containing various words and spaces
34 this is a another sentence containing various words and spaces

cat file2.txt

2 this is sentence1file2
6 this is sentence2file2
54 this is sentence3file2

I would like to join these 2 files. The result should look as follows :

cat joinedfile.txt

2 this is sentence1file2
6 this is sentence2file2
15 this is a sentence containing various words and spaces
34 this is a another sentence containing various words and spaces
54 this is sentence3file2

==> so the joined file must be sorted on the first number. Any ideas how this can be achieved ?