N = input()
L =[]
for i in range(0,N):
    first = raw_input()
    split_first = first.split()
    X = int(split_first[0])
    Y = int(split_first[1])
    L.append(Y-X)
L=set(L)

if len(set(L)) == 1:
    L = list(L)
    if L[0] >= 1:
        print L[0]
    else:
        print -1
else:
    print -1