#pragma GCC optimize(2)
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef __int128_t int128;
typedef pair<int, int> pii;
#define iofast ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define lowbit(x) (x&(-x))
#define inv(x) qpow(x,mod-2)
#define debug(x) cout << (#x) << " = " << x << endl
#define range(x) (x).begin(), (x).end()

const int iINF = 0x3f3f3f3f;
const ll llINF = 0x3f3f3f3f3f3f3f3f;
const ll mod = 998244353;

/*---------------------------------------*/
int n, m;
vector<ll>v;

bool check(ll mid)
{
    vector<ll>now(n + 5);
    ll ans = 0;
    for (int i = 1;i <= n;i++) {
        now[i] += v[i];
        ans++;
        if (now[i] >= mid)
            continue;
        ll cnt = (mid - now[i]) / v[i] + bool((mid - now[i]) % v[i]);
        now[i + 1] += cnt * v[i];
        ans += cnt * 2;
    }

    return ans <= m;
}

void solve()
{
    cin >> n >> m;
    v.clear();
    v.resize(n + 5);
    for (int i = 1;i <= n;i++)
        cin >> v[i];

    ll l = 0, r = 1e18;
    while (l < r) {
        ll mid = (l + r + 1) >> 1;
        if (check(mid))
            l = mid;
        else
            r = mid - 1;
    }

    cout << l << '\n';
}

/*---------------------------------------*/

int main()
{
    iofast;
    int t = 1;
    cin >> t;
    while (t--)
        solve();

    return 0;
}