#include <bits/stdc++.h>
using namespace std;

const int MOD = (1e9+7);
void printmat(const vector<vector<int>>& mat) {
    for (auto row : mat) {
        for (auto elem : row)
            cout << elem << " ";
        cout << endl;
    }
}
void printv(const vector<int>& v) {
    for (auto elem : v)
        cout << elem << " ";
    cout << endl;
}
void printd(const deque<int>& v) {
    for (auto elem : v)
        cout << elem << " ";
    cout << endl;
}
void printvp(const vector<pair<int,int>>& vp) {
    for (auto pr : vp) {
        cout << pr.first << ", " << pr.second;
        cout << endl;
    }
}
void printvs(const vector<set<int>>& vs) {
    for (auto row : vs) {
        for (auto elem : row)
            cout << elem << ", ";
        cout << endl;
    }
}
void printht(const unordered_map<int, int>& ht) {
    for (auto elem : ht)
        cout << elem.first << " : " << elem.second << endl;
}
void printmp(const map<int, int>& ht) {
    for (auto elem : ht)
        cout << elem.first << " : " << elem.second << endl;
}
void printst(const set<int>& st) {
    for (auto elem : st)
        cout << elem << " ";
    cout << endl;
}

bool isPrime(long long n) {
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
    if (n % 2 == 0 || n % 3 == 0)
        return false;
    for (long long i = 5; i * i <= n; i = i + 6)
        if (n % i == 0 || n % (i + 2) == 0)
            return false;
    return true;
}

map<long long, long long> primeFactors(long long n) {
    map<long long, long long> ans;
    while (n % 2 == 0) {
        ans[2]++;
        n = n/2;
    }
    for (long long i = 3; i*i <= (n); i = i + 2) {
        while (n % i == 0) {
            ans[i]++;
            n = n/i;
        }
    }
    if (n > 2)
        ans[n]++;
    return ans;
}

int find_f(const vector<int>& uf, int i) {
    while (uf[i]!=i)
        i = uf[i];
    return i;
}
bool union_f(vector<int>& uf, vector<int>& sz, int a, int b) {
    a = find_f(uf, a);
    b = find_f(uf, b);
    //cout << "a, b = " << a << ", " << b << endl;
    if (a==b) return false;
    if (sz[a] < sz[b]) {
        //cout << "sz[a], sz[b] = " << sz[a] << ", " << sz[b] << endl;
        //cout << "a, b = " << a << ", " << b << endl;
        swap(a,b);
        //cout << "a, b = " << a << ", " << b << endl;
    }
    sz[a] += sz[b];
    uf[b] = a;
    return true;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);    
    int T=1, caseIdx=0;
    //cin >> T;
    while (T--) {
        //caseIdx++;
        int n, ans=-1;
        cin >> n;
        set<int> st;
        for (int i=0; i<n; i++) {
            int x, y;
            cin >> x >> y;
            st.insert(y-x);
        }
        if (st.size()==1 && *st.begin()>0) 
            ans = *st.begin();
        cout << ans << endl;
        //cout << "Case #" << caseIdx << ": " << s << endl;
    }
}