#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int maxn=3e5+10;
const ll inf=2e18+10;
ll a[maxn], b[maxn];
bool check(ll x, ll n, ll m){
ll qtd=0;
for(int i=1;i<=n;i++){
if(b[i]>=a[i]) qtd+=(x+b[i]-1)/b[i];
else{
if(a[i]*m>=x) qtd+=(x+a[i]-1)/a[i];
else{
ll y=x-a[i]*m;
qtd+=m;
qtd+=(y+b[i]-1)/b[i];
}
}
}
return qtd<=n*m;
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
ll n, m; cin >> n >> m;
for(int i=1;i<=n;i++) cin >> a[i];
for(int i=1;i<=n;i++) cin >> b[i];
ll l=0, r=inf;
while(l<r){
ll mid=((r-l+1)/2)+l;
if(check(mid,n,m)) l=mid;
else r=mid-1;
}
cout << l << endl;
}
