#include <bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define pii pair <int , int>
#define arr3 array <int , 3>
using namespace std;
const int INF = 1e18;
const int maxn = 1e6 + 7;
int n , m , a[maxn] , b[maxn];
bool check(int mid)
{
int cnt = 0;
for(int i = 1; i <= n; i++)
{
if(a[i] > b[i])
{
int c1 = min(m , (mid - 1)/a[i] + 1);
cnt += c1;
if(c1*a[i] < mid) cnt += (mid - c1*a[i] - 1)/b[i] + 1;
}
else cnt += (mid-1)/b[i] + 1;
}
return (cnt <= (n*m));
}
void solve()
{
cin >> n >> m;
for(int i = 1; i <= n; i++) cin >> a[i];
for(int i = 1; i <= n; i++) cin >> b[i];
int l = 0 , r = 1e18+2 , ans = 0;
while(l <= r)
{
int mid = (l+r)>>1;
if(check(mid))
{
ans = mid;
l = mid + 1;
}
else r = mid - 1;
}
cout << ans << '\n';
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
solve();
return 0;
}
