It looks like you need to add a delimiter argument to your function call. From the help documentation for importdata:
importdata(FILENAME, DELIM) interprets DELIM as the column separator in
ASCII file FILENAME. DELIM must be a string. Use '\t' for tab.
If your data is tab-separated, try
a = importdata('Input.txt','\t')
In the future, you can find the help documentation for any function by typing
help functionName