n = int(input())

ans = []
for i in range(n):
    y, a = [int(i) for i in input().split()]
    x = a - y
    ans.append(x)
    
b = set(ans)

if len(b) == 1:
    if min(b) <= 0:
        print(-1)
    else:
        print(min(b))
else:
    print(-1)