#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int nx=3e5+5;
ll n, m, a[nx], b[nx], l=1, r=1e18;
bool check(ll x)
{
// return 1 when we can do x
ll cnt=0;
for (int i=1; i<=n; i++)
{
// calculate minimum classes needed in order to make comprehension level of i'th subject >= x
// a[i]>=b[i]
ll cnta=0, cntb=0, curx=x; // amount of classes that we take A
if (x%a[i]==0) cnta=x/a[i];
else cnta=x/a[i]+1;
if (cnta<=m) // purely study in class is enough
{
cnt+=cnta;
}
else
{
curx-=m*a[i];
if (curx%b[i]) cntb=curx/b[i];
else cntb=curx/b[i]+1;
cnt+=m+cntb;
}
if (cnt>n*m) return 0;
}
return 1;
}
int main()
{
cin.tie(NULL)->sync_with_stdio(false);
cin>>n>>m;
for (int i=1; i<=n; i++) cin>>a[i];
for (int i=1; i<=n; i++) cin>>b[i], a[i]=max(a[i], b[i]);
while (l<r)
{
ll md=(l+r+1)/2;
if (check(md)) l=md;
else r=md-1;
}
cout<<l;
}
