#include <iostream>
#include <vector>
#define loop(i,a,b) for(int i=a;i<b;i++)
using namespace std;
long long A[300050], B[300050];
int main(){
ios::sync_with_stdio(false); cin.tie(0);
long long N, M;
cin >> N >> M;
loop(i,0,N) cin >> A[i];
loop(i,0,N) cin >> B[i];
loop(i,0,N) A[i] = max(A[i], B[i]);
long long ok = 0, ng = 1LL << 60;
auto check = [&](long long x) -> bool {
long long day = 0;
loop(i,0,N){
if(x <= A[i] * M) day += (x - 1) / A[i] + 1;
else day += M + (x - A[i] * M - 1) / B[i] + 1;
if(day > N * M) return 0;
}
return 1;
};
while(ng - ok > 1){
long long mid = (ng + ok) / 2;
if(check(mid)) ok = mid;
else ng = mid;
}
cout << ok << endl;
return 0;
}
