#include<bits/stdc++.h>
#define int long long
#define deg(a) cout << #a << " = " << a << "\n";
#define de(a) cout << #a << " = " << a << " ";
#define x first
#define y second
using namespace std;
const int N=1e5+10;
int a[N];
signed main()
{
    int n;
    cin>>n;
    for(int i=1;i<=n;i++)cin>>a[i];
    for(int i=1;i<=n/2+1;i++)
    {
        int j=n+1-i;
        int mi=min(a[i],a[j]);
        a[i]-=mi;
        if(j!=i)a[j]-=mi;
    }
    int res=0;
    for(int i=1;i<=n;i++)
    {
        res+=a[i];
        a[i+1]-=min(a[i+1],a[i]);
    }
    cout<<res<<endl;
    return 0;
}