N=int(input())
x = []
y = []
for i in range(N):
    x1,y1=[int(i) for i in input().split()]
    x.append(x1)
    y.append(y1)

l=[]

for i in range(N):
    l.append(y[i]-x[i])

def multi_diff(es):
    return es == [es[0]] * len(es) if es else False

if multi_diff(l):
    if l[0]>0:
        print(l[0])
    else:
        print(-1)
else:
    print(-1)