import java.util.*;
import java.io.*;
import static java.util.Arrays.*;
import static java.lang.Math.*;

public class No0185 {
    
    static final Scanner in = new Scanner(System.in);
    static final PrintWriter out = new PrintWriter(System.out,false);

    static void solve() {
        int n = in.nextInt();
        int res = 0;
        for (int i=0; i<n; i++) {
            int x = in.nextInt();
            int y = in.nextInt();
            if (res == -1) continue;
            if (i == 0)
                res = y - x;
            else if (res != y - x) res = -1;
        }
        out.println(res <= 0 ? "-1" : res);
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();

        solve();
        out.flush();

        long end = System.currentTimeMillis();
        //trace(end-start + "ms");
        in.close();
        out.close();
    }

    static void trace(Object... o) { System.out.println(deepToString(o));}
}