#include "bits/stdc++.h"
 
using namespace std;
 
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
 
#define INF 0x3f3f3f3f
#define LNF (ll) 0x3f3f3f3f3f3f3f3f 
 
#define maxn 100010

ll a[maxn], def[maxn];

void solve() {
    ll n, m; cin >> n >> m;

    for (int i = 0; i < n; ++ i) cin >> a[i];

    ll l = 0, r = LNF;
    
    while (l < r) {
        ll md = (l + r + 1) >> 1;

        ll stp = 0;
        memset(def, 0, sizeof def);
        for (int i = 0; i < n; ++ i) {
            if (md <= def[i]) {
                stp += i != n-1;
            } else {
                ll cnt = (md - def[i] + a[i] - 1) / a[i];

                stp += cnt * 2 - 1;
                if (stp > m) break;
                def[i+1] = (a[i+1] * (cnt - 1));
            }
        }

        if (stp <= m) {
            l = md;
        } else {
            r = md - 1;
        }
    }

    cout << l - 1 << '\n';
}

int main() {
#ifdef TEST
    freopen("zz.in", "r+", stdin);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int t; cin >> t;
    while (t --) solve();
    return 0;
}