#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>
typedef long long llong;
const int MAXN = 300000 + 10;
const llong INF = 1e18;
llong n, m;
llong a[MAXN], b[MAXN];
llong achieved[MAXN];
bool check(llong value)
{
__int128 freeB = 0;
for (int i = 1 ; i <= n ; ++i)
{
llong currLections = std::min(m, (value / a[i] + ((value % a[i]) > 0)));
if (value <= m * a[i]) freeB += m - currLections;
else
{
llong moreLections = (value - m * a[i]) / b[i] + (((value - m * a[i]) % b[i]) > 0);
freeB -= moreLections;
}
}
return freeB >= 0;
}
void solve()
{
llong min = INF;
llong max = 0;
for (int i = 1 ; i <= n ; ++i)
{
min = std::min(min, m * a[i]);
max = std::max(max, m * a[i]);
}
llong l = min, r = max + 1, mid;
while (l < r - 1)
{
mid = (l + r) / 2;
if (check(mid)) l = mid;
else r = mid;
}
std::cout << l << '\n';
}
void read()
{
std::cin >> n >> m;
for (int i = 1 ; i <= n ; ++i)
{
std::cin >> a[i];
}
for (int i = 1 ; i <= n ; ++i)
{
std::cin >> b[i];
a[i] = std::max(a[i], b[i]);
}
}
void fastIO()
{
std::ios_base :: sync_with_stdio(0);
std::cout.tie(nullptr);
std::cin.tie(nullptr);
}
int main()
{
fastIO();
read();
solve();
return 0;
}
