Subtract the next numbers Description

Subtract the next P numbers from a N number. The next number of N is N + 1.

Look at the examples to get what I mean.

Examples:

Input: N=2,P=3
Calculate: n - (n+1) - (n+2) - (n+3)     //Ending with 3, because P=3
Calculate: 2 -  2+1  -  2+2  - 2+3       //Replacing N with 2 from Input
Calculate: 2 -  3    -  4    - 5
Output: -10


Input: N=100,P=5
Calculate: n - (n+1) - (n+2) - (n+3) - (n+4) - (n+5)
Calculate: 100-  101 -  102  -  103  -  104  - 105
Output: -415


Input: N=42,P=0
Calculate: n
Calculate: 42
Output: 42


Input: N=0,P=3
Calculate: n - (n+1) - (n+2) - (n+3)
Calculate: 0 -  1    -  2    -  3
Output: -6


Input: N=0,P=0
Calulate: n
Calculate: 0
Output: 0


Input:

N: Integer, positive, negative or 0

P: Integer, positive or 0, not negative

Output:

Integer or String, leading 0 allowed, trailing newline allowed

Rules:


No loopholes
This is code-golf, so shortest code in bytes wins
Input and Output must be as described
Python 2, 26 24 23 bytes
-2 bytes thanks to @Adnan (replace p*(p+1)/2 with p*-~p/2)
-1 byte thanks to @MartinEnder (replace -p*-~p/2 with +p*~p/2

lambda n,p:n-p*n+p*~p/2

Tests are on ideone