N = int(input())
import sys
x = [0] * N
y = [0] * N
for i in range(N):
    x[i],y[i] = map(int,input().split())
a = y[0] - x[0]
for i in range(1,N):
    if a != y[i] -x[i]:
        print(-1)
        exit()
if a <= 0:
    print(-1)
    exit()
print(a)