#include <bits/stdc++.h>
using namespace std;
#define int __int128
int n,m,a[300005],b[300005],nev[300005];
int nrparti(int x,int y)
{
if (x % y == 0)
return x / y;
return x / y + 1;
}
bool pot(int x)
{
int extra = 0;
for (int i = 1; i <= n; i++)
{
nev[i] = x;
if (a[i] <= b[i])
extra += m;
else
{
if (x <= m * a[i])
{
extra += m - nrparti(x,a[i]);
nev[i] = 0;
}
else
{
nev[i] = x - m * a[i];
}
}
}
for (int i = 1; i <= n; i++)
extra -= nrparti(nev[i],b[i]);
if (extra >= 0)
return true;
return false;
}
int solvem1()
{
int ans = 1e9;
for (int i = 1; i <= n; i++)
ans = min(ans,max(a[i],b[i]));
return ans;
}
int solve()
{
int st = 0,pas = 1ll << 60;
while (pas != 0)
{
if (pot(st + pas) == true)
st += pas;
pas /= 2;
}
return st;
}
void citire()
{
long long N,M;
cin >> N >> M;
n = N,m = M;
for (int i = 1; i <= n; i++)
{
long long X;
cin >> X;
a[i] = X;
}
for (int i = 1; i <= n; i++)
{
long long X;
cin >> X;
b[i] = X;
}
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
citire();
cout << (long long)solve();
/*srand(0);
n = 10;
m = 1;
while (true)
{
for (int i = 1; i <= n; i++)
a[i] = b[i] = 1 + rand() % 10;
if (solvem1() != solve())
{
for (int i = 1; i <= n; i++)
cout << a[i] << ' ';
cout << endl;
for (int i = 1; i <= n; i++)
cout << b[i] << ' ';
}
}*/
return 0;
}
/**
10 1
3 1 1 2 6 1 1 1 1 1
3 1 1 2 6 1 1 1 1 1
**/
