determine the difference between the sum of the 1000 first even positive integers and the sum of the first 1000 odd positive integers.

this is how I thought:

2+4+6+8+...+1000

an=a1+(n-1)*d

1000=2+(n-1)*2

1000=2+2n-2

1000-2+2=2n

n=500

sum=(n/2)*(a1+an)=(500/2)*(2+1000)

sum=250500

Then I tought

1+3+5+7+...+1000

an=a1+(n-1)*d

1000=1+(n-1)*2

1000=1+2n-2

1000=-1+2n

1001=2n

n=500,5

sum=(500,5/2)*(1+1000)

sum=250500,25

250500,25-The answer is 1000. How did I Count wrong??250500=0,25