Integer Constants

  The general form of an integer constant is a sign (plus or minus) followed by a string of one or more digits. All other characters (except blanks) are prohibited. If the number is positive the plus sign is optional. Here are some examples of valid integer constants:
-100 42 0 +1048576
It is easier to read a large number if its digits are marked off in groups of three: traditionally the comma (or in some countries the dot) is used for this purpose. The blank can be used in the same way in Fortran programs (but not in data files):
-1 000 000 000
Note that this number, although conforming to the rules of Fortran, may be too large in magnitude to be stored as an integer on some systems.