#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll n, m;
ll l = 0, r = 4e18, mid;
ll ceill(ll a, ll b) {return (a+b-1)/b;}
ll getsum(ll a, ll own, ll classi, ll mid) {
ll b = ceill(mid - a*classi, own); b = max(b, 0ll);
return a + b;
}
int main() {
cin.tie(0) -> sync_with_stdio(0);
cin >> n >> m;
vector<ll> a(n), b(n);
for(ll &e:a) cin >> e;
for(ll &e:b) cin >> e;
for(int i=0;i<n;i++) a[i] = max(a[i], b[i]);
while(l <= r) {
mid = (l+r) >> 1;
ll sum = 0, NM = n*m;
for(int i=0;i<n;i++) {
//ll A = min(m, mid / a[i]);
//ll res = ceill(A*(b[i] - a[i]) + mid, b[i]);
ll res = min({getsum(0, b[i], a[i], mid), getsum(min(m, mid/a[i]), b[i], a[i], mid), getsum(min(m, mid/a[i]+1), b[i], a[i], mid)});
res = min(res, getsum(min(m, max(0ll, mid/a[i]-1)), b[i], a[i], mid));
sum += res;
if(sum > NM) break;
//cout << res << ' ';
} //cout << '\n';
//cout << mid << " -> " << sum << '\n';
if(sum <= n*m) l = mid + 1;
else r = mid - 1;
}
cout << r;
}
