In the first example you can choose a sequence b = {1, 2}, so the sum  is equal to 7 (and that's 3 after taking it modulo 4).

In the second example you can choose a sequence b = {3}.

AC code

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

int n,m;
vector<int> A,X,Y;

void gen(int l, int r, vector<int> &X)
{
    X.push_back(0);
    for (int i=l; i<=r; ++i)
    {
        int s = X.size();
        for (int x=0; x<s; ++x)
            X.push_back((A[i]+X[x])%m);
    }
}

int main()
{
    cin >> n >> m;
    A.resize(n);
    for (int i=0; i<n; ++i)
        cin >> A[i];

    int mid = n/2;
    gen(0,mid,X);
    gen(mid+1,n-1,Y);
    sort(Y.begin(),Y.end());

    int res = 0;
    for (auto ix=X.begin(); ix!=X.end(); ++ix)
        res = max(res,*ix + *(upper_bound(Y.begin(),Y.end(),m-1-*ix)-1));
    cout << res << endl;
    return 0;
}