import java.io.*;
import java.util.*;
import java.util.stream.*;

// 処理
class Process {
    private int[] X;
    private int[] Y;

    Process(int[] X, int[] Y) {
        this.X = X;
        this.Y = Y;
    }

    int getResult() {
        int difference = -1;
        for(int i = 0; i < X.length; i++) {
            if(X[i] >= Y[i]) {
                return -1;
            }
            
            if(difference == -1) {
                difference = (Y[i] - X[i]);
                continue;
            }
            
            if((Y[i] - X[i]) != difference) {
                return -1;
            }
        }

        return difference;
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        var bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        var printWriter    = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        // 入力
        int N = Integer.parseInt(bufferedReader.readLine().trim());
        
        var X = new int[N];
        var Y = new int[N];
        for(int i = 0; i < N; i++) {
            int[] input = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();
            X[i] = input[0];
            Y[i] = input[1];
        }

        // 出力
        printWriter.println((new Process(X, Y)).getResult());

        bufferedReader.close();
        printWriter.close();
    }
}