N = int(input())
D = [list(map(int, input().split())) for x in range(N)]
L = [D[x][1]-D[x][0] for x in range(N)]
p = L[0]

for n in range(N):
    if L[n-1] == L[n]:
        pass
    else: p = -1

if p > 0:
    print(p)
else:
    print('-1')