Help with 279B - Codeforces

#	User	Rating
1	Radewoosh	3611
2	jiangly	3597
3	Benq	3593
4	ecnerwala	3584
5	orzdevinwang	3570
6	cnnfls_csy	3569
7	tourist	3565
8	maroonrk	3487
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10	Rebelz	3409

#	User	Contrib.
1	maomao90	174
2	awoo	164
3	adamant	162
4	TheScrasse	159
5	nor	158
6	maroonrk	156
7	-is-this-fft-	151
8	SecondThread	147
9	orz	146
10	pajenegod	145

Problem

My approach

The total time taken to read all books is sum. Then for every iteration if the sum is greater than the free time you can either remove the first book or the last book in the series, it's better to remove the book with the higher read duration for optimal solution (It is optimal because, since we have to remove a book, it's better to remove the one with the higher read duration to have more potential time for reading other books). If there are any doubts regarding the approach, refer to the code.

Code

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#define ll long long

using namespace std;

int main(void) {
    ll n, t; cin >> n >> t;
    vector<ll> a(n);
    ll sum = 0;
    for (ll i = 0; i < n; ++i) {
        cin >> a[i];
        sum += a[i];
    }
    ll lo = 0;
    ll hi = n - 1;
    while (sum > t && hi >= lo) {
        if (a[lo] > a[hi]) {
            sum -= a[lo];
            lo++;
        } else {
            sum -= a[hi];
            hi--;
        }
    }
    cout << hi - lo + 1 << endl;
}

Issue

Obviously my approach or program (or both) are incorrect, but I'm having a hard time finding what is wrong. I would appreciate any help. Thanks in advance :)