// ~~ icebear ~~
#include <bits/stdc++.h>
using namespace std;
#define int ll
typedef long long ll;
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
template<class T>
bool minimize(T &a, const T &b) {
if (a > b) return a = b, true;
return false;
}
template<class T>
bool maximize(T &a, const T &b) {
if (a < b) return a = b, true;
return false;
}
#define FOR(i,a,b) for(int i=(a); i<=(b); ++i)
#define FORR(i,a,b) for(int i=(a); i>=(b); --i)
#define REP(i, n) for(int i=0; i<(n); ++i)
#define RED(i, n) for(int i=(n)-1; i>=0; --i)
#define MASK(i) (1LL << (i))
#define BIT(S, i) (((S) >> (i)) & 1)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define task "icebear"
/*END OF TEMPLATE. ICEBEAR AND THE CAT WILL WIN VOI26 */
const int MOD = 1e9 + 7;
const int inf = 1e9 + 27092008;
const ll INF = 1e18 + 27092008;
const int N = 3e5 + 5;
int n, m, a[N], b[N];
bool check(ll mid) {
ll cnt = 0;
FOR(i, 1, n) {
ll x = min((ll)m, (mid + a[i] - 1) / a[i]);
ll left = max(0ll, mid - x * a[i]);
ll y = (left + b[i] - 1) / b[i];
cnt += x + y;
}
return cnt <= 1LL * n * m;
}
void init(void) {
cin >> n >> m;
FOR(i, 1, n) cin >> a[i];
FOR(i, 1, n) cin >> b[i], maximize(a[i], b[i]);
}
void process(void) {
ll low = 1, high = 1e18, res = 0;
while(low <= high) {
ll mid = (low + high) >> 1;
if (check(mid)) res = mid, low = mid + 1;
else high = mid - 1;
}
cout << res;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
int tc = 1;
// cin >> tc;
while(tc--) {
init();
process();
}
return 0;
}
