it's 1,000,000:
the sum is 1 + 3 + 5 + ... 1999999
= 1 + 2 + ... + 2000000 - 2*(1 + ... + 1000000)
= 2000000*2000001/2 - 2*(1000000*1000001 / 2)
= 1000000*(2000001 - 1000001) = 1000000000000 (one million million)
and dividing this by one million to get the average,
you get one million