#include <iostream>
using namespace std;
int main()
{
unsigned int koncerty;
long double suma=0;
cin>>koncerty;
long double tablica[koncerty];
for (int i=0; i<koncerty; i++)
{
cin>>tablica[i];
suma+=tablica[i];
}
if (tablica[0]<0)
suma=suma-tablica[0];
if (tablica[koncerty-1]<0)
suma=suma-tablica[koncerty-1];
cout<<suma<<endl;
return 0;
}
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