using namespace std;
#include <bits/stdc++.h>
#define BEGIN ios_base::sync_with_stdio(0);cin.tie(0);
#define END return EXIT_SUCCESS;
#define FOR(I,A,B) for(int (I)=(A);(I)<(B);++(I))
#define ALL(C) (C).begin(),(C).end()
inline void solve()
{
	int N;cin>>N;
	vector<int>S(N);
	int T=0;
	FOR(i,0,N)
	{
		int a,b;cin>>a>>b;
		S[i]=a+4*b;
		T=max(T,S[i]);
	}
	int M=0;
	FOR(i,0,N)
	{
		M+=(T-S[i]);
	}
	if(M%2==0)cout<<(M/2)<<endl;
	else cout<<(-1)<<endl;

}
int main(int argc,char**argv)
{
	BEGIN
	solve();
	END
}