#include <bits/stdc++.h>
using namespace std;
#define print(l) for(auto i:l) cout<<i<<" ";cout<<endl;
#define input(t,l,n) vector<t>l(n);for(int i = 0;i<n;i++)cin>>l[i];
#define int long long
#define pb push_back
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define all(l) l.begin(),l.end()
#define pii pair<int,int>
#define fi first
#define se second
const int M = 1e9+7;
const int inf = 1e18;
int bp(int x, int y, int p){
int res = 1;
x = x % p;
while (y > 0) {
if (y & 1)
res = (res * x) % p;
y = y >> 1;
x = (x * x) % p;
}
return res;
}
int MI(int n, int p){
return bp(n, p - 2, p);
}
int mul(int x,int y, int p){
return x * 1ull * y % p;
}
int di(int x,int y, int p){
return mul(x, MI(y, p), p);
}
int n , m , k , q;
void solve(int testcase_number){
cin>>n>>k;
input(int,a,n);
input(int,b,n);
int ans = inf;
for(int i = 0;i<n;i++){
ans = min(ans,max(a[i],b[i]));
}
int l = 0;
int r = 3e18;
while(r-l>1){
int m = (l+r)/2;
int ec = 0;
int rec = 0;
for(int i = 0;i<n;i++){
if(a[i] >= b[i]){
if(a[i]*k >= m){
int c = m/a[i];
while(c*a[i] < m){
c++;
}
ec+=(k-c);
}
else{
int t = m-a[i]*k;
int c = t/b[i];
while(c*b[i] < t){
c++;
}
rec+=c;
}
}
else{
if(b[i]*k >= m){
int c = m/b[i];
while(c*b[i] < m){
c++;
}
ec+=(k-c);
}
else{
int t = m-b[i]*k;
int c = t/b[i];
while(c*b[i] < t){
c++;
}
rec+=c;
}
}
}
if(rec<=ec){
l = m;
}
else{
r = m;
}
}
cout<<l<<endl;
}
signed main(){
ios::sync_with_stdio(0);//DO NOT USE IN INTERACTIVE
cin.tie(0), cout.tie(0);
cout << fixed<<setprecision(9);
int t = 1;
// cin>>t;
for(int i = 1;i<=t;i++){
solve(i);
}
}
