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)