def main():
    N = int(input())
    XY = []
    for i in range(N):
        x, y = map(int, input().split())
        XY.append(y-x)
    if XY.count(XY[0])==len(XY) and XY[0]>0:
        print(XY[0])
    else:
        print(-1)
    
if __name__ == '__main__':
    main()