import sys

def cal(x):
    a, b = map(int,x.split())
    if b > a:
        return b - a 
    else:
        print(-1)
        sys.exit(0)

n = int(input())
i = cal(input())

for _ in range(n-1):
    j = cal(input())
    if i != j:
        print(-1)
        sys.exit(0)

print(i)