n = int(input())
p = list(map(int, input().split()))


while True:
    tmp = []
    if len(p) % 2 == 1:
        tmp.append(p[-1])
    for i in range(n//2):
        tmp.append(p[2*i]*p[2*i+1])
    p = tmp[0:]
    n = len(p)
    if n == 1:
        ans = p[0]
        break

while True:
    if len(str(ans)) == 1:
        print(ans)
        break
    tmp = 0
    for i in str(ans):
        tmp += int(i)
    ans = tmp