#include <bits/stdc++.h>
#define FOR(i,l,r) for(int i = l; i <= r; ++i)
#define FOD(i,r,l) for(int i = r; i >= l; --i)
#define task "CAVERN"
#define int long long
using namespace std;
const int maxn = 5555;
const int inf = 1e18;
#define _58693_
int n, m;
int a[maxn], b[maxn];
deque < int > qa, qb;
int mxa[maxn], mxb[maxn];
int ans = 0;
int check(int d)
{
//cerr << d << endl;
while(!qa.empty()) qa.pop_back();
while(!qb.empty()) qb.pop_back();
int res = inf;
FOD(i,m,0)
{
while(!qa.empty() && qa.front() > i + d) qa.pop_front();
while(!qa.empty() && a[qa.back()] < a[i]) qa.pop_back();
qa.push_back(i);
while(!qb.empty() && qb.front() > i + d) qb.pop_front();
while(!qb.empty() && b[qb.back()] < b[i]) qb.pop_back();
qb.push_back(i);
if(i <= m - d) mxb[i] = b[qb.front()];
if(i <= m - d) mxa[i] = a[qa.front()];
if(i <= m - d)
{
res = min(res, n - mxa[i] - mxb[i]);
if(res < 0) return 0;
}
}
return res;
}
main()
{
#ifdef _58693_
freopen(task".inp","r",stdin);
freopen(task".out","w",stdout);
#endif // _58693_
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n >> m;
FOR(i,1,m) cin >> a[i];
FOR(i,1,m) cin >> b[i];
a[0] = 0;
b[0] = 0;
FOR(i,1,m)
{
//cout << i << " ";
ans = max(ans,i*check(i));
}
cout << ans;
return 0;
}
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