#include <bits/stdc++.h>

#define rep(i, l, r) for(int i = (int)l; i < (int)r; i++)
#define all(v) v.begin(), v.end()
#define sz(v) (int)v.size()
using namespace std;

using ll = long long;
using pll = pair<ll, ll>;
using pii = pair<int, int>;
using ld = long double;

typedef vector<ll> vl;
typedef vector<int> vi;

template<class T>
void out(const vector<T> &v) {
    for (auto x: v)cout << x << " ";
    cout << endl;
}

const ll linf = 1e15 + 10;

void solve() {
    
    int n;ll m;cin >> n >> m;
    vector<int> a(n);for(auto &x:a)cin >> x;

    auto check = [&](vector<ll> c) -> ll{
        ll cost = 0;
        rep(i, 0, n - 1) {
            cost += 1;
            if (c[i] > 0){
                cost += (c[i] - 1) * 2;
                c[i+1] -= c[i] - 1;
            }
        }
        if (c[n-1] > 0) {
            cost += 1 + 2 * (c[n-1] - 1);
        }
        return cost;
    };

    ll ok = 0, ng = 1e18 + 10;
    while (ng - ok > 1) {
        ll mid = ok + (ng - ok) / 2;
        vector<ll> c(n);rep(i, 0, n)c[i] = (mid - 1) / a[i] + 1;
        // cout << mid << " " << check(c) << endl;
        if (check(c) <= m) ok = mid;
        else ng = mid;
    }
    cout << ok << endl;
}

int main() {


    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int T; cin >> T;
    while(T--) {
        solve();
    }
    
}