#include <stdio.h>
#include <iostream>
#include <string>
using namespace std;

int main(){
	
	int n;
	cin >> n;

	int ans = 0;

	int v[1004];
	for(int i=0; i<n; i++){
		cin >> v[i];
	}

	int i = 0;
	while(i<n){
		int a = 0;
		int b = 0;
		int c = 0;
		int d = 0;
		
		a = v[i];
		b = v[i+1];
		c = v[i+2];
		d = v[i+3];

		if(a+c >= b+d){
			ans += a;
			i++;
			//cout << a;
		}
		else{
			ans += b;
			i += 2;
			//cout << b;
		}
	}
	
	cout << ans << endl;

	return 0;
}