#include <iostream>
using namespace std;
int main() {
int i=0;
int tab[101];
do{
cin >> tab[i];
i++;
}while(tab[i-1]!=-1);
i--;
if(i%2!=0){
cout<<tab[i/2];
}
else{
cout<<(tab[i/2-1]+tab[i/2])/2.0;
}
return 0;
}
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