import sys

def input(): return sys.stdin.readline().strip()
def list2d(a, b, c): return [[c] * b for i in range(a)]
def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]
def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]
def ceil(x, y=1): return int(-(-x // y))
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]
def Yes(): print('Yes')
def No(): print('No')
def YES(): print('YES')
def NO(): print('NO')
sys.setrecursionlimit(10 ** 9)
INF = 10 ** 18
MOD = 10 ** 9 + 7
EPS = 10 ** -10

N = INT()
A = [0] * (N-1)
B = [0] * (N-1)
for i in  range(N-1):
    A[i], B[i] = MAP()

smin = sum(A)
smout = sum(B)
mxout = 0
for i, (a, b) in enumerate(zip(A, B)):
    mxout += min(a, smout - b)

# ans = 0
# i = 0
# while 1:
#     cnt = smin + i - smout
#     if 0 <= cnt and smout - i <= mxout:
#         if cnt <= smin:
#             ans += 1
#         else:
#             break
#     i += 1
mn = max(smout - smin, smout - mxout, 0)
mx = smout
ans = max(mx - mn + 1, 0)
print(ans)