N = int(input())

x, y = map(int, input().split())

ans = y - x

if ans <= 0:
    print(-1)
    exit()

for i in range(N - 1):
    
    x, y = map(int, input().split())
    
    if y - x != ans:
        
        print(-1)
        
        exit()
        
print(ans)