N = int(input())

ans_set = set()
A = 0
pairs = []

for _ in range(N):
    X, Y = map(int, input().split())
    pairs.append((X, Y))

for X, Y in pairs:
    Z = Y - X
    if Z > 0:
        ans_set.add(Z)
    else:
        A = -1

if A == -1:
    print(-1)
elif len(ans_set) == 1:
    print(ans_set.pop())
else:
    print(-1)