#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main(){
    int N,X,Y;
    cin >> N;
    vector<int> M(N);
    for(int i=0;i<N;i++){
        cin >> X >> Y;
        M[i]=Y-X;
    }
    sort(M.begin(),M.end());
    if(M[0]==M[N-1] && M[0]>0)cout << M[0] << endl; 
    else cout << -1 << endl;
    return 0;
}