#include<bits/stdc++.h>
using namespace std;
int a[100010],n,ans=0,i;
int main()
{
cin>>n;
for(i=1;i<=n;i++)cin>>a[i];
for(int i=n;i>=1;i--){
if((a[i]+ans&1)==0)ans=(a[i]+ans)>>1;
else ans=(a[i]+ans>>1)+1;
}
cout<<ans;
}