#include <iostream>
#include <fstream>
using namespace std;
ifstream f("vase.in");
ofstream g("vase.out");
int main()
{
int n,nr,l,s=0
,i;
char c;
f>>n;
for(i=1;i<=n;i++)
{
f>>nr>>c;
s=s+nr;
}
f>>l;
g<< s/2;
return 0;
}
