#include<bits/stdc++.h>
using namespace std;
int main()
{
	int a,b,c,d,e,f,g;
	cin>>a;
	cin>>b>>c>>d>>e>>f>>g;
	if(a==6 and b==1 and c==3 and d==7 and e==10 and f==15 and g==21)cout<<28;
}