[twitter][*] Missing Number

You have n - 1 numbers from 1 to n. Your task is to find the missing number.

I.e.
n = 5
v = [4, 2, 5, 1]
The result is 3.

This is easy like sum them up and subtracted by (1+n)*n/2

My Code:

int findmissing(int a[], int n){

    int missing = 0;

    for(int i = 0; i < n; ++i){

        missing += a[i];

    }

    missing = (1+n+1)*(n+1)/2 - missing;

    return missing;

    }