Ball Distribution

#include <bits/stdc++.h>

using namespace std;

#define ll     long long
#define _test   int _TEST; cin>>_TEST; while(_TEST--)

int main()
{
    _test
    {
        ll int n, m;
        cin>>n>>m;

        vector<ll int> a(m);
        for(auto &e: a)     cin>>e;

        ll int tot = accumulate(a.begin(), a.end(), 0ll);

        ll int mmin = (m-1)*1ll*n;

        cout<<max(0ll, tot-mmin)<<"\n";
    }
}