n = int(input())
x = []
y = []

minus = 0
res = 0

for i in range(n):
    xi,yi=[int(i) for i in input().split()]
    x.append(xi)
    y.append(yi)
    if y[i] - x[i] <= 0:
        minus = 1
        res = -1

delta = y[0] - x[0]
if minus == 0:
    for i in range(n):
        if delta != y[i] - x[i]:
            res = -1
    if res == 0:
        res = delta
        
print(res)