Help with an algorithm design problem

[PJ]

The problem states: You are given an unsorted list of n−1 distinct integers from the range 1 to n. Write a linear-time algorithm to find the missing integer.

For example, you have the list of integers from 1 - 5, with the number 4 missing. So you could have: 3, 1, 5, 2. You are to find that the missing number is 4, in 5 or fewer comparisons.

Is it possible to solve this problem in linear time O(n)? I can certainly think of a way to do it in n^2 time, but I'm stuck on linear time...

Any help or hint would be appreciated.

[ROI]

Are you allowed to do computations? You can sum up the values and get the difference from n(n+1)/2.