Add them all up, subtract that from the actual sum of 1-250 and voila. Optimization bonus: you don't need to do the second sum, there's a closed form (I don't remember what it is, but I do remember Gauss came up with it as a kid).
but you can get the formula by imagining the numbers as piles of pennies and then completing them to make a square and then splitting down the diagonal
*
**
***
*..
**.
***
\.. \
*\. + \
**\ \
so it's 1/2 * n * n (ie half the square) plus an extra 1/2 for each one on the diagonal.
so it's n * n/2 + n/2 = (n+1) * n/2
1727321173475
could be 1, 72, 73, 211, ... or 17, 27, 32, 117, ... or 172, ...