N=int(input())
X,Y=[],[]
for i in range(N):x,y=map(int,input().split());X.append(x);Y.append(y)
s=Y[0]-X[0]
if s<1:print(-1);exit()
for i in range(1,N):
	if Y[i]-X[i]!=s:print(-1);exit()
print(s)