Here are my two submission for problem : 1158A - The Party and Sweets

1. using accumulate function: 300047140

2. using loop: 300047827

1st one giving wrong answer. Previously, the same thing happened with sqrt, and then I used binary search to find sqrt. So after this WA, I used a loop to get the sum, and now it's giving the correct answer. So where has this accumulate function gone wrong?