#include <cstdio>
using namespace std;
template<typename T>
T read(){
	char c=getchar();T x=0;
	while(c<48||c>57) c=getchar();
	do x=(x<<1)+(x<<3)+(c^48),c=getchar();
	while(c>=48&&c<=57);
	return x;
}
typedef long long ll;
const int N=1000003;
int n;ll m;
int a[N];
ll b[N];
bool calc(ll lim){
	ll res=0;
	for(int i=1;i<=n+1;++i) b[i]=0;
	for(int i=1;i<=n;++i){
		b[i]+=a[i];++res;
		if(b[i]<lim){
			ll t=(lim-b[i]+a[i]-1)/a[i];
			b[i]+=t*a[i];
			b[i+1]+=(t-1)*a[i];
			res+=t+t-1;
			if(res>m) return 0;
		}
	}
	return res<=m;
}
int main(){
	int tc=read<int>();
	while(tc--){
		n=read<int>();m=read<ll>();
		for(int i=1;i<=n;++i) a[i]=read<int>();
		ll l=0,r=1e17;
		while(l<r){
			ll mid=(l+r)>>1;
			if(calc(mid)) l=mid+1;
			else r=mid;
		}
		printf("%lld\n",l-1);
	}
	return 0;
}