C# Codegolf: Sum of 7s - Sum of 5s

Question

Let's see who can calculate the difference between multiples of 7 and multiples of 5. Shortest answer wins! (Look at my code of the same name for more rules)

Answer 1

Oh, now you've got me thinking non-linearly.

Answer 2

64 characters:
int n=1000,m=500,x=n/5,y=m/7;
Console.Write(y*++y*7/2-x*++x*5/2);