N = int(input())
A = []
B = []
for _ in range(N - 1):
    a, b = map(int, input().split())
    A.append(a)
    B.append(b)
S = sum(A)
T = sum(B)
res = 0
for i in range(min(S, T) + 1):
    p = max(S, T) + i
    for j in range(N - 1):
        if min(A[j], B[j]) * 2 > p:
            break
    res += 1
print(res)