#include <iostream>
#include <vector>
#include <list>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <cmath>
#include <cstring>

using namespace std;

#define FOR(x,y) for(int x = 0;x < (y);x++)
#define LLI long long int
#define FORR(x,arr) for(auto& x:arr)
#define ALL(a) (a.begin()),(a.end())

#define _L(x) cout<<(x)<<endl
//#define _L(x) ;

template<int um> class UF {
public:
    vector<int> _parent,_rank;
    UF()
    {
        _parent=_rank=vector<int>(um,0);
        for(int i=0;i<um;i++)
        {
            _parent[i]=i;
        }
    }
    
    int Find(int x)
    {
        if(_parent[x] == x)
        {
            return x;
        }
        _parent[x] = Find(_parent[x]);
        return _parent[x];
    }
    
    int Union(int x,int y)
    {
        int xRoot = Find(x);
        int yRoot = Find(y);
        if(_rank[xRoot]>_rank[yRoot])
        {
            _parent[xRoot] = yRoot;
           return yRoot;
        }
        if(_rank[xRoot]<_rank[yRoot])
        {
            _parent[yRoot] = xRoot;
            return xRoot;
        }
        if(xRoot != yRoot)
        {
            _parent[yRoot] = xRoot;
            _rank[xRoot]++;
            return xRoot;
        }
        return xRoot;
    }
};

int dp[(1 << 16) + 1];

int main() {
    LLI n;
    cin >> n;
    
    int x, y;
    cin >> x >> y;
    if(y <= x)
    {
        cout << -1 << endl;
        return 0;
    }
    int answer = y - x;
    for(int i = 0; i < n - 1;i++)
    {
        cin >> x >> y;
        if(answer != (y-x))
        {
            cout << -1 << endl;
            return 0;
        }
    }
    cout << answer << endl;
    return 0;
}