#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
int n=3,i;
int niz[n];
for (i=0;i<n;i++)
cin>>niz[i];
sort(niz,niz+n) ;

cout<<endl;
for (i=0;i<n;i++)
{
if (niz[1]-niz[0]==niz[2]-niz[1])
cout<<niz[2]+niz[2]-niz[1];
if (niz[2]-niz[1] == 2*(niz[1]-niz[0]))
cout<< (niz[2]+niz[1]) /2;
if (niz[1]-niz[0] == 2*(niz[2]-niz[1]))
cout<<(niz[0]+niz[1]) /2;break;
}

return 0;
}