#include<bits/stdc++.h>
#define ll long long
#define mk make_pair
using namespace std;
ll n,h[200005];
ll cgt(ll t,pair<ll,ll>s)
{
	ll res=t;
	if(s.first<=t)res+=min(s.second,t)-s.first;
	return res;
}
bool check(ll t)
{
	ll yt=t;
	pair<ll,ll>s=mk(0,0);
	for(ll i=1;i<=n;++i)
	{
		if(cgt(t,s)<h[i])
		{
			t-=(h[i]-cgt(t,s));
			s=mk(0,0);
		}
		else
		{
			ll res=t;
			if(min(t,s.first)>=h[i]) res=h[i];
			else if(cgt(min(t,s.second),s)>=h[i])res=min(t,s.second)-(cgt(min(t,s.second),s)-h[i])/2;
			else res=s.second+h[i]-cgt(min(t,s.second),s);
			s=mk(res,t);
			t=yt;
		}
		if(t<0)return 0;
	}
	return t==yt;
}
int main()
{
	ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
	cin>>n;
	for(ll i=1;i<=n;++i)cin>>h[i];
	ll l=0,r=1000000000,res=1000000000;
	while(l<=r)
	{
		ll mid=l+r>>1;
		if(check(mid))res=mid,r=mid-1;
		else l=mid+1;
	}
	cout<<res;
}