To find the sum of the given arithmetic series, we need to determine the pattern and the number of terms.

The pattern of the series is: +1, -3, +5, -7, ...

We can see that the terms alternate between adding and subtracting odd numbers. The first term is +1, the second term is -3, the third term is +5, and so on.

The common difference between consecutive terms is 2. Each term is obtained by adding 2 to the previous term, then changing the sign.

To find the number of terms, we can use the formula for the nth term of an arithmetic series:

nth term = first term + (n-1) * common difference

We need to find the value of n when the nth term is -33:

-33 = 1 + (n-1) * 2

Simplifying the equation:

-33 = 1 + 2n - 2
-33 = 2n - 1
-32 = 2n
n = -16

Since the number of terms cannot be negative, we discard this solution.

Therefore, there is no whole number of terms that will result in the last term being -33.

Hence, the sum of the series 1-3+5-7+...-33 is undefined.