#include using namespace std; #define all(c) (c).begin(),(c).end() #define rrep(i,n) for(int i=(int)(n)-1;i>=0;i--) #define REP(i,m,n) for(int i=(int)(m);i<(int)(n);i++) #define rep(i,n) REP(i,0,n) #define iter(c) __typeof((c).begin()) #define tr(it,c) for(iter(c) it=(c).begin();it!=(c).end();it++) #define pb(a) push_back(a) #define pr(a) cout<<(a)<> #define F first #define S second #define ll long long bool check(int n,int m,int x,int y){return x>=0&&x=0&&y P; int main() { int n; cin >> n; int ans=-1; rep(i,n) { int x,y; cin >> x >> y; int z=y-x; if(z>=0 && (ans==z || ans==-1)) ans=z; else ans=-2; } if(ans==-2) ans=-1; cout << ans << endl; return 0; }