N = int(input())
if N == 2:
    print(1)
    exit()

A_sum, B_sum = 0, 0
for i in range(N - 1):
    A, B = map(int,input().split())
    A_sum += A
    B_sum += B

print(B_sum + 1 - max(0, B_sum - A_sum))