#undef _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define int long long
#define pb push_back
#define fi first
#define si second
#define ar array
typedef pair<int,int> pi;
typedef tuple<int,int,int> ti;
void debug_out() {cerr<<endl;}
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {cerr<<" "<<to_string(H);debug_out(T...);}
#define debug(...) cerr<<"["<<#__VA_ARGS__<<"]:",debug_out(__VA_ARGS__)
int N, M;
int A[300010], B[300010];
int good(int x) {
int cnt = 0;
for (int i = 1; i <= N; ++i) {
int opt = max(A[i], B[i]); // treat study as course -> doenst steal from other subjects
int a = 0, b = 0;
if (x <= opt * M) {
a = (x + opt - 1) / opt;
} else {
a = M;
b = (x - M * opt + min(A[i], B[i]) - 1) / B[i];
}
cnt += a + b;
// debug(a, b);
}
return cnt <= M * N;
}
signed main() {
fast;
cin >> N >> M;
for (int i = 1; i <= N; ++i) cin >> A[i];
for (int i = 1; i <= N; ++i) cin >> B[i];
int lo = 0, hi = 1e18;
while (lo + 1 < hi) {
int mid = (lo + hi) / 2;
if (good(mid)) lo = mid;
else hi = mid;
}
cout << lo;
return 0;
}
