#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>
#include <array>
#include <iomanip>
using namespace std;
#define int long long
const int N = 1e5 + 10,INF = 1e17;
#define endl '\n'
int n,m,T;
int a[N];

bool check(int x)
{
    int res = 0,last = 0;
    for(int i = 1;i <= n;i++)
    {
        if(x > last)
        {
            int cnt = max(1ll,(x + a[i] - 1 - last) / a[i]);
            res += 1 + 2 * (cnt - 1);
            last = (cnt - 1) * a[i + 1];
        }

    }
    return res <= m;
}

void solve()
{
    cin >> n >> m;
    for(int i = 1;i <= n;i++) cin >> a[i];
    int l = 0,r = INF;
    while(l < r)
    {
        int mid = (l + r + 1) >> 1;
        if(check(mid)) l = mid;
        else r = mid - 1;
    }
    cout << l << endl;
}


signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> T;
    while(T--)
    {
        solve();
    }
}