The sum of the numbers from -1 to -100 can be calculated using the formula for the sum of an arithmetic series:

Sum = (n/2)(first term + last term)

In this case, the first term is -1 and the last term is -100. The number of terms, n, can be calculated using the formula:

n = (last term - first term)/common difference + 1

The common difference in this case is -1, as each term decreases by 1. Substituting the values into the formulas:

n = (-100 - (-1)) / (-1) + 1
= (-100 + 1) / (-1) + 1
= -99 / -1 + 1
= 99 + 1
= 100

Now, we can calculate the sum:

Sum = (n/2)(first term + last term)
= (100/2)(-1 + (-100))
= 50(-1 - 100)
= 50(-101)
= -5050

Therefore, the sum of the numbers from -1 to -100 is -5050.