#include using namespace std; typedef long long ll; typedef unsigned long long ull; typedef pair pii; #define FOR(i,a,n) for(int i=a;i<(int)(n);i++) #define REP(i,n) FOR(i,0,n) #define ALL(a) (a).begin(),(a).end() #define MP(a,b) make_pair(a,b) #define PB(a) push_back(a) #define F first #define S second const int INF = 2000000000; const int DX[4]={0,1,0,-1}, DY[4]={-1,0,1,0}; struct P{int x;int y;P(int X=0,int Y=0){x=X;y=Y;}}; int main() { int N; cin >> N; vector a(N); vector b(N); vector top(N); int m=0; REP(i,N) { cin >> a[i] >> b[i]; top[i] = a[i]+4*b[i]; m = max(m, top[i]); } bool b1=false, b2=false; REP(i,N) { if(top[i] % 2 == 1) b1=true; else b2=true; } if(b1 && b2) { cout << -1 << endl; return 0; } int ans = 0; while(true) { bool b = false; REP(i,N) { if(m>top[i]) { b=true; while (true) { if(m<=top[i]) break; top[i]+=2; ans++; } m=top[i]; } } if(!b) break; } cout << ans << endl; return 0; }