You are referring to:
[tex]\sum\limits_{i=1}^{n}{i} = \frac{n(n+1)}{2}[/tex]
You can solve your problem by calculating
[tex]\sum\limits_{i=1}^{525}{i} = \frac{525\cdot526}{2} = 138075 [/tex]
and then subtracting the first bit:
[tex]\sum\limits_{i=1}^{19}{i} = 190[/tex]
So 138075 - 190 = 137885
