#include <bits/stdc++.h>
#define int long long
using namespace std;
const int nmax = 3e5 + 1;
int a[nmax], b[nmax], n, m;
bool check(int nrm) {
int cnt = 0;
for(int i = 1; i <= n; i ++) {
//cout << "Suntem la " << i << endl;
int nrai = nrm / a[i];
if(nrm % a[i] != 0) {
nrai ++;
//cout << "uWu(" << a[i] << " " << nrm << ") ";
}
// cout << " nrai = " << nrai << " si al final ";
int cntn = 0;
if(nrai <= m) {
cnt += nrai;
cntn += nrai;
} else {
nrai = m;
int rm = nrm - a[i] * nrai;
cnt += rm / b[i];
cnt += nrai;
cntn += rm / b[i];
cntn += nrai;
if(rm % b[i] != 0) {
cnt ++;
cntn ++;
}
}
//cout << "Cnt: " << cntn << endl;
}
// cout << "Cnt final: " << cnt << endl;
return (cnt <= n * m);
}
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
cin >> n >> m;
for(int i = 1; i <= n; i ++) {
cin >> a[i];
}
for(int i = 1; i <= n; i ++) {
cin >> b[i];
a[i] = max(a[i], b[i]);
}
int l = 0, r = 1e18, ans = 0;
while(l <= r) {
int mid = (l + r) / 2;
if(check(mid) == true) {
ans = mid;
l = mid + 1;
} else {
r = mid - 1;
}
}
cout << ans;
return 0;
}
