import math
import fractions
#import sys
#input = sys.stdin.readline

def make_divisors(n):
    divisors = []
    for i in range(1, int(n**0.5)+1):
        if n % i == 0:
            divisors.append(i)
            if i != n // i:
                divisors.append(n//i)

    # divisors.sort()
    return divisors

def ValueToBits(x,digit):
    res = [0 for i in range(digit)]
    now = x
    for i in range(digit):
        res[i]=now%2
        now = now >> 1
    return res

def BitsToValue(arr):
    n = len(arr)
    ans = 0
    for i in range(n):
        ans+= arr[i] * 2**i
    return ans

def ZipArray(a):
    aa = [[a[i],i]for i in range(n)]

    aa.sort(key = lambda x : x[0])
    for i in range(n):
        aa[i][0]=i+1
    aa.sort(key = lambda x : x[1])
    b=[aa[i][0] for i in range(len(a))]
    return b

def ValueToArray10(x, digit):
    ans = [0 for i in range(digit)]
    now = x
    for i in range(digit):
        ans[digit-i-1] = now%10
        now = now //10
    return ans

def Zeros(a,b):
    if(b<=-1):
        return [0 for i in range(a)]
    else:
        return [[0 for i in range(b)] for i in range(a)]

def AddV2(v,w):
    return [v[0]+w[0],v[1]+w[1]]

dir4 = [[1,0],[0,1],[-1,0],[0,-1]]


def clamp(x,y,z):
    return max(y,min(z,x))

class Bit:
    def __init__(self, n):
        self.size = n
        self.tree = [0] * (n + 1)
 
    def sum(self, i):
        s = 0
        while i > 0:
            s += self.tree[i]
            i -= i & -i
        return s
 
    def add(self, i, x):
        while i <= self.size:
            self.tree[i] += x
            i += i & -i
            

class UnionFind:
    def __init__(self, n):
        self.par = [i for i in range(n+1)]
        self.rank = [0] * (n+1)

    # 検索
    def find(self, x):
        if self.par[x] == x:
            return x
        else:
            self.par[x] = self.find(self.par[x])
            return self.par[x]

    # 併合
    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)
        if self.rank[x] < self.rank[y]:
            self.par[x] = y
        else:
            self.par[y] = x
            if self.rank[x] == self.rank[y]:
                self.rank[x] += 1

    # 同じ集合に属するか判定
    def same_check(self, x, y):
        return self.find(x) == self.find(y)


'''
def cmb(n, r, p):
    if (r < 0) or (n < r):
        return 0
    r = min(r, n - r)
    return fact[n] * factinv[r] * factinv[n-r] % p

p = 2
N = 10 ** 6 + 2
fact = [1, 1]  # fact[n] = (n! mod p)
factinv = [1, 1]  # factinv[n] = ((n!)^(-1) mod p)
inv = [0, 1]  # factinv 計算用

for i in range(2, N + 1):
    fact.append((fact[-1] * i) % p)
    inv.append((-inv[p % i] * (p // i)) % p)
    factinv.append((factinv[-1] * inv[-1]) % p)

'''

#a = list(map(int, input().split()))
#list(map(int, input().split()))

#################################################
#################################################
#################################################
#################################################


#28-

a = input()
b = input()
c = input()
now = 'a'
while(True):
    #print(a,b,c)
    if(now=='a'):
        if(len(a)==0):
            print('A')
            exit()
        now = a[0]

        a = a[1:]
        
    elif(now=='b'):
        if(len(b)==0):
            print('B')
            exit()
        now = b[0]

        b = b[1:]
        
    else:

        if(len(c)==0):
            print('C')
            exit()
        now = c[0]
        c = c[1:]
    #print('next:',now)






















































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