#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=300005;
ll n,m,A[N+2],B[N+3];
ll ch(ll mid){
ll tot=0;
ll limit = n * m; // Total available slots across all courses
for(int i=1;i<=n;i++){
ll want=mid;
ll tclas=0,tself=0;
// If attending class is better than self-study
if(A[i]>B[i]){
tclas=want/A[i];
if(tclas*A[i]<want)++tclas;
tclas=min(tclas,m); // Use up to m native slots for this course
want-=tclas*A[i];
tot+=tclas;
}
// Use any remaining slots (native or from other courses) for self-study
if(want>0){
tself=want/B[i];
if(tself*B[i]<want)++tself;
tot+=tself;
}
// CHANGE 1: Prevent integer overflow of tot and exit early
if(tot > limit) return limit + 1;
}
return tot;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
if (!(cin >> n >> m)) return 0;
for(int i=1;i<=n;i++) cin >> A[i];
for(int i=1;i<=n;i++) cin >> B[i];
// CHANGE 2: Range should go up to 2e18 because N*M slots
// and high B[i] can result in very high levels.
ll l=1, r=2e18, ans=0;
while(l<=r){
ll mid=l+(r-l)/2;
if(ch(mid)<=n*m){
ans=mid;
l=mid+1;
}
else r=mid-1;
}
cout << ans << endl;
return 0;
}
