#include <iostream>
#include<vector>
using namespace std;
int n;
int main()
{
    cin>>n;vector<int>h(n);
    for(int i=0;i<n;i++)cin>>h[i];
    int ans=0;
    for(int i=n-1;i>=0;i--)
    {
        ans=(ans+h[i]+1)/2;
    }
    cout<<ans<<'\n';

    return 0;
}