Language: C++

#include <bits/stdc++.h> 
#include <boost/multiprecision/cpp_int.hpp> 
using namespace boost::multiprecision;
using namespace std;
#define ll long long 
#define mp make_pair
#define pb push_back
#define se second
#define fi first 
#define upp 1e18
#define mod 1000000007

void solve() {
    int1024_t i, j, k, l, t, n, x, y, r,q,m,n1;
    int1024_t a[200007];
    cin>>n;
    ll sum=0;
    for(i=0;i<n;i++) cin>>a[i];
    for(i=0;i<n;i++) {
        sum+=a[i];
    }
    ll product=0;
    for(i=0;i<n;i++) {
        sum-=a[i];
        product+=a[i]*sum;
    }
    cout<<product%mod;
}




















int main() {
    #ifndef ONLINE_JUDGE
        freopen("input1.txt", "r", stdin);
        freopen("output1.txt", "w", stdout);
    #endif
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    solve();
}
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//  MMMMMMMMMMMMMMNyrrrRrWMMmzOrzdNyrrrZNvrwQWWfb....(r~~_J>_.....~.~~-dpfpffpfpppWHHMNdMMMMMMMMMMMMMMMM
//  MMMMMMMMMMMMMMMNrtrStdMMMNxzrzMNmrrrdNWfVfffP._-.(~_J!............JpfpffpfpfffpppWNppppHMMMMMMMMMMMM
//  MMMMMMMMMMMMMMMMNOtXtdMMMMMNxzZMMNmgHyVVVVVW%..?/(.,(x-..........(HfffpffpffpffWHHffpffppfVyyWMMMMMM

Compiler output:
a.cc:2:10: fatal error: boost/multiprecision/cpp_int.hpp: No such file or directory
    2 | #include <boost/multiprecision/cpp_int.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.