#include <stdio.h>
int main(){
    //input
    int N;
    scanf("%d",&N);

    int X[1000];
    int Y[1000];
    for (int i = 0;i < N;i++){
        scanf("%d %d",&X[i],&Y[i]);
    }

    //calculation
    int answer;
    for (int i = 0;i < N;i++){
        int tmp = Y[i]-X[i];
        if (tmp > 0){
            if (i == 0){
                answer = tmp;
            }else{
                if (answer != tmp){
                    answer = -1;
                    break;
                }
            }
        }else{
            answer = -1;
            break;
        }
    }

    //output
    printf("%d\n",answer);
    return 0;
}