# No.185 和風
n = int(input())
x = [0] * n
y = [0] * n
for i in range(n):
    x[i], y[i] = [int(j) for j in input().split()]
    
is_equal = True
answer = y[0] - x[0]

if answer <= 0:
    print(-1)
elif n == 1:
    print(answer)
else:
    for i in range(1, n):
        if y[i] - x[i] != answer:
            is_equal = False
            break
    
    print(answer if is_equal else -1)