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 len(Z) == 1:
    print(Z)
else:
    print(-1)