import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
while 1:
n, m = mii()
if n == m == 0:
break
l = [ii() for _ in range(n - 2)] + [m]
l.sort()
p(n * (n + 1) // 2 - sum(l))
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()