#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #define REP(i,n) for(long (i)=0;(i)<(n);(i)++) #define FOR(i,a,b) for(long (i)=(a);(i)<(b);(i)++) #define RREP(i,a) for(long (i)=(a)-1;(i)>=0;(i)--) #define FORR(i,a,b) for(long (i)=(a)-1;(i)>=(b);(i)--) #define MOD 1000000007 #define PI acos(-1.0) #define DEBUG(C) cout< #define PLL pair #define ALL(a) (a).begin(),(a).end() #define SORT(a) sort((a).begin(),(a).end()) typedef long long LL; typedef unsigned long long ULL; using namespace std; int main(void){ int N; cin>>N; long ans=0; long a,b; vector v(N); REP(i,N){ cin>>a>>b; v[i]=a+(4*b); } SORT(v); REP(i,N-1) ans+=v[N-1]-v[i]; if(ans&1) ans=-1; else ans/=2; cout<