#include <bits/stdc++.h>
#define endl '\n'
#define int long long
using namespace std;
void solve(){
    int n,m;
    cin>>n>>m;
    vector<int>a(n+1);
    for (int i = 1; i <=n ; ++i) {
        cin>>a[i];
    }
    auto check=[&](int x){
        auto b=a;
        for (int i = 1; i <=n ; ++i) {
            b[i]=(x-1)/a[i]+1;
        }
        b.push_back(0);
        int sum=0;
        for (int i = 0; i <=n ; ++i) {
            if(sum>m)return false;
            if(b[i]<=0){
                if(i==n)continue;
                b[i+1]--;
                sum++;
                continue;
            }
            else{
                sum+=b[i]*2;
                b[i+1]-=b[i];
                if(i==n)continue;
                if(i<n-1){
                    b[i+1]--;
                    sum++;
                }
                else if(i==n-1 and b[i+1]>0){
                    b[i+1]--;
                    sum++;
                }
            }
        }
        return sum<=m;
    };
    int l=0,r=2e18;
    int ans=-1;
    while(l<=r){
        int mid=l+r>>1;
        if(check(mid)){
            l=mid+1;
            ans=mid;
        }
        else r=mid-1;
    }

    cout<<ans<<endl;

}
signed main() {
    ios::sync_with_stdio(false),cin.tie(0);
    int t;
    cin>>t;
    while ( t-- ){
        solve();

    }
    return 0;
}