N = int(input())

L = []
for _ in range(N):
    x, y = map(int, input().split())
    if y - x <= 0:
        L += ['NG']
    else:
        L += [y - x]
L = list(set(L))

print([-1, *L][len(L) == 1 and 'NG' not in L])