Search:

What would happen if you started with

sum[thirteen] = multiply * (thirteen + 1);

?

The first value of thirteen is 0, so sum[0] = multiply * 0 = 0 the way you have it. (I think Mr. 777 said...

I'm guessing this is what you meant.

for (int i=0; i < 400; i++)
{
sum[i] = 13 * (i+1);
}