#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;


int main() {

    int N;
    cin >> N;
    int ans = -1;
    for (int i = 0; i < N; ++i) {
        int X, Y;
        cin >> X >> Y;

        if (Y - X > 0) {
            ans = Y - X;
        }

    }

    cout << ans << endl;
    return 0;
}