#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(void)
{
	int n, *a, i, j, sum=0;
	scanf("%d", &n);
	a = (int *)malloc(sizeof(int)*n);
	for( i = 0; i < n; i++) {
		scanf("%d", &a[i]);
	}

	for(i = 0; i < n; ) {
		if( i+1 == n ) {
			sum += a[i];
			i += 2;
		} else if ( i+2 == n) {
			( a[i] >= a[i+1] ) ? sum += a[i] : sum += a[i+1];
			i += 3;
		} else if ( i+3 == n) {
			( a[i]+a[i+2] >= a[i+1] ) ? sum = sum+a[i]+a[i+2] : sum += a[i+1];
			i += 4;
		} else {
			if ( a[i]+a[i+2] >= a[i]+a[i+3] && a[i]+a[i+2] >= a[i+1]+a[i+3] ) {
				sum = sum + a[i] + a[i+2];
				i += 4;
			} else if ( a[i]+a[i+3] >= a[i+1]+a[i+3] ) {
				sum = sum + a[i] + a[i+3];
				i += 5;
			} else {
				sum = sum + a[i+1] + a[i+3];
				i += 5;
			}
		}
	}

	printf("%d\n", sum);

	return 0;
}