# coding: utf-8

def main():

    n = int(input())
    
    result = -1

    for i in range(n):
        x, y = [int(j) for j in input().split()]
        z = y - x
        if z <= 0:
            break
        if result == -1:
            result = z
        else:
            if result != z:
                result = -1
                break

    print(result) 

if __name__=='__main__':
    main()