int cmpval (const void *a, const void *b) {
    return *(int *)a - *(int *)b;
}
int arrayPairSum(int* nums, int numsSize){
    int sum = 0, i;
    qsort(nums, numsSize, sizeof(int), cmpval);
    for(i = 0; i < numsSize; i = i + 2)
        sum = sum + nums[i];
    return sum;
}