#include <bits/stdc++.h>
#define N 300005
using namespace std;
typedef long long ll;

int n, m, a[N], b[N];

bool ok (ll x)
{
    ll ans=0;
    for (int i=1; i<=n; i++) {
        ll cur=((x-1)/a[i])+1;
        if (cur<=m) { ans+=m-cur; }
        else { ans-=((x-1ll*m*a[i]-1)/b[i])+1; }
    }
    return ans>=0;
}

int main ()
{
    // freopen("0test.in", "r", stdin);
    // freopen("0test.out", "w", stdout);

    scanf("%d %d", &n, &m);
    for (int i=1; i<=n; i++) { scanf("%d", &a[i]); }
    for (int i=1; i<=n; i++) { scanf("%d", &b[i]); a[i]=max(a[i], b[i]); }

    ll l=0, r=1000000000000000000, ans=0;
    while (l<=r) {
        ll md=(l+r)>>1;
        if (ok(md)) { ans=md; l=md+1; }
        else { r=md-1; }
    }
    printf("%lld\n", ans);

    return 0;
}