#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main() {
    int n,k,a[10000],m,c=0,j;
    cin>>n;
    m=n/2;
   for(int i=0;i<n;i++){
       cin>>a[i];
   }
        j=1;
    while(m--){
        c+=a[j];
        j=j+2;
    }
        cout<<c;
    
        
        
    
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */   
    return 0;
}