#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define int ll
#define pii pair<int, int>
#define f first
#define s second
#define REP(i,n) for (int i = 0; i<n; ++i)
#define REP1(i,n) for (int i = 1; i<=n; ++i)
#define SZ(x) (int)((x).size())
#define ALL(x) (x).begin(), (x).end()
#define pb push_back
#define MX(a,b) a = max(a,b)
#define MN(a,b) a = min(a,b)
#ifdef BALBIT
#define bug(...) cerr<<"#"<<__LINE__<<" "<<#__VA_ARGS__<<" - ", _do(__VA_ARGS__)
template <typename T> void _do( T && x) {cerr<<x<<endl;}
template <typename T, typename ...S> void _do( T && x, S && ...y) {cerr<<x<<", "; _do(y...);}
#else
#define bug(...)
#define endl '\n'
#endif // BALBIT
const int maxn = 3e5+5;
ll a[maxn], b[maxn];
int n,m;
bool can(ll X) {
ll pool = 0;
REP(i,n) {
ll keep = min(X / a[i], m);
ll need = (X - a[i] * keep + b[i]-1) / b[i];
ll g1 = m-keep-need;
keep = min((X+a[i]-1) / a[i], m);
need = (max(0ll, X - a[i] * keep)
+ b[i]-1) / b[i];
ll g2 = m-keep-need;
pool += max(g1, g2);
bug(i, g1, g2);
}
bug(X, pool);
return pool >= 0;
}
signed main(){
ios::sync_with_stdio(0), cin.tie(0);
bug(1,2);
cin>>n>>m;
REP(i,n) cin>>a[i];
REP(i,n) cin>>b[i], MX(a[i], b[i]);
ll l = 0, r = 4e18;
while (l!=r) {
ll mid = (l+r)/2;
if (can(mid)) {
l = mid+1;
}else{
r = mid;
}
}
cout<<l-1<<endl;
}
