START:
PRINT "Enter two integers between -1000 and +1000 separated by at least one space: "
INPUT "> "; n$ '                   | Enter two numbers with at least one space between.
n$ = _TRIM$(n$) '                  | TRIM any leading or trailing spaces.
bpos = INSTR(n$, " ") '            | Find the first space between the two numbers.
a = VAL(LEFT$(n$, bpos - 1)) '     | Parse the first number from the input string.
b = VAL(_TRIM$(MID$(n$, bpos))) '  | Parse the second number from the input string.
IF (a < -1000 OR a > 1000) OR (b < -1000 OR b > 1000) THEN
  PRINT "A number is outside of limit (-1000 to +1000). Try again.": PRINT
  GOTO START '                     | Check both number are within prescribed limit.
END IF
a$ = LTRIM$(STR$(a)) '             | Clean up both numbers and the sum for better printing.
b$ = LTRIM$(STR$(b)) '             | "
sum$ = LTRIM$(STR$(a + b)) '       | "
PRINT "The sum of the two integers a + b = "; a$; " + "; b$; " = "; sum$