It is difficult to add text inside a text file. The easiest thing to do is to create a new text file and output everything to it, copying from the original file.
infid = fopen('YourFile.txt', 'rt');
outfid = fopen('YourNewFile.txt', 'wt');
fprintf( outfid, '20\n' ); %the text you are adding at the beginning
while true
thisline = fgetl(infid); %read line from input file
if ~ischar(thisline); break; end %reached end of file
fprintf( outfid, '%s\n', thisline ); %write the line to the output file
end
fclose(infid);
fclose(outfid);
