#include <iostream> #include <iomanip> #include <algorithm> #include <math.h> #include <string> #include <list> ///////// #define REP(i, x, n) for(int i = x; i < n; i++) #define rep(i,n) REP(i,0,n) #define P(p) cout<<(p)<<endl; ///////// typedef long long LL; typedef long double LD; ///////// using namespace::std; ///////// int main(void){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed;// //cout << setprecision(6);// int N;//[2,1000] cin>>N; int a[1000],b[1000]; rep(i,N){ cin>>a[i]>>b[i];//[0,10^7] } /////////// int count = 0; int temp = 0; int max = a[0] + 4*b[0]; for(int i=1;i<N;++i){ temp = a[i] + 4*b[i]; if(max < temp ){ if((temp-max)%2 == 1){ P(-1);return 0; }else{ count += i*(temp-max)/2; max = temp; } } else if( max == temp){ } else if( max > temp){ if( (max-temp)%2 == 1){ P(-1);return 0; } else{ count += (max-temp)/2; } } } P(count); return 0; }