class Solution:
    def solve(self, a):
        flag = True
        for i in range(len(a) - 1):
            if a[i + 1] - a[i] != 1:
                flag = False
                return a[i] + 1

        if flag == True:
            if a[0]!=0:
                return 0
            else:
                return a[-1]+1