To simplify the expression \( 99 - 97 + 95 - 93 + \ldots - 1 \), we first identify the pattern in the sequence of terms.

The sequence of terms starts from 99 and decreases by 2 for each term until reaching 1. The full sequence is:

\[
99, -97, 95, -93, \ldots, -1
\]

This series alternates between positive and negative numbers.

To separate the terms, we can rewrite it like this:

\[
(99 + 95 + 91 + 87 + \ldots + 3) - (97 + 93 + 89 + 85 + \ldots + 1)
\]

**Step 1: Identify the positive terms.**

The positive terms are:

\[
99, 95, 91, 87, 83, 79, 75, 71, 67, 63, 59, 55, 51, 47, 43, 39, 35, 31, 27, 23, 19, 15, 11, 7, 3
\]

This is an arithmetic series where:
- First term \( a = 99 \)
- Last term \( l = 3 \)
- Common difference \( d = -4 \)

To find the number of terms \( n \), we use the formula for the last term of an arithmetic series:

\[
l = a + (n-1)d
\]

Substituting the known values:

\[
3 = 99 + (n-1)(-4)
\]
\[
3 = 99 - 4(n - 1)
\]
\[
3 - 99 = -4(n - 1)
\]
\[
-96 = -4(n - 1)
\]
\[
96 = 4(n - 1)
\]
\[
n - 1 = 24 \implies n = 25
\]

Now, we can calculate the sum of these positive terms using the sum formula for an arithmetic series:

\[
S_n = \frac{n}{2} \times (a + l) = \frac{25}{2} \times (99 + 3) = \frac{25}{2} \times 102 = 25 \times 51 = 1275
\]

**Step 2: Identify the negative terms.**

The negative terms are:

\[
-97, -93, -89, -85, -81, -77, -73, -69, -65, -61, -57, -53, -49, -45, -41, -37, -33, -29, -25, -21, -17, -13, -9, -5, -1
\]

This is another arithmetic series where:
- First term \( a = -97 \)
- Last term \( l = -1 \)
- Common difference \( d = 4 \)

Again, we find the number of terms \( m \):

\[
l = a + (m-1)d
\]

Substituting the values:

\[
-1 = -97 + (m-1)(4)
\]
\[
-1 + 97 = 4(m - 1)
\]
\[
96 = 4(m - 1)
\]
\[
m - 1 = 24 \implies m = 25
\]

Now, calculate the sum of these negative terms:

\[
S_m = \frac{m}{2} \times (a + l) = \frac{25}{2} \times (-97 - 1) = \frac{25}{2} \times (-98) = 25 \times -49 = -1225
\]

**Step 3: Combine the results.**

Finally, we find the total sum of all the terms:

\[
\text{Total} = S_n + S_m = 1275 - 1225 = 50
\]

Thus, the result of the expression \( 99 - 97 + 95 - 93 + \ldots - 1 \) is

\[
\boxed{50}.
\]