#include <bits/stdc++.h>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define pii pair<long long,int>
#define ff first
#define ss second
#define mp make_pair
#define pb push_back
#define STL(i, x) for(auto &i:x) cout << i << " "; cout << "\n";
using namespace std;
long long a[300050], b[300050], c[300050];
/*ostream& operator<<(ostream &os, pii XXX){
cout << "[" << XXX.ff << ", " << XXX.ss << "]";
}*/
void db(){
cout << "\n";
}
template<class T, class ...U>
void db(T XXX, U ...YYY){
cout << XXX; db(YYY...);
}
int main(){
long long n, m, all;
cin >> n >> m;
all = n * m;
loop(i,0,n) cin >> a[i];
loop(i,0,n) cin >> b[i];
loop(i,0,n) c[i] = max(a[i], b[i]);
long long l = 1, r = (long long)(9e18), mid;
long long cost, classes;
while(l != r){
mid = (l + r + 1) >> 1;
classes = 0;
loop(i,0,n){
cost = mid;
if(b[i] >= a[i]){
classes += ((cost-1) / b[i] + 1);
}
else{
classes += min(((cost-1) / a[i] + 1), m);
cost -= min(((cost-1) / a[i] + 1), m) * a[i];
if(cost > 0)
classes += ((cost-1) / b[i] + 1);
}
}
if(classes > n * m){
r = mid - 1;
}
else{
l = mid;
}
}
cout << l << "\n";
return 0;
}
/*
3 3
19 4 5
2 6 2
*/
