#include <iostream>
#include <fstream>
#include <string> 
#include <cmath>  
#include <cstdlib>
#include <ctime>
#include <vector>
#include <list>  
#include <algorithm>
#include <numeric>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <deque>
#include <functional>

using namespace std;
using ll = long long;
using ull = unsigned long long;

#define FOR(i, m, n) for(int i = int(m);i < int(n);i++)
#define REFOR(i, m, n) for(int i = int(n - 1);i >= int(m);i--)
#define REP(i,n) for(int i = 0; i < int(n); i++)
#define REREP(i,n) for(int i = int(n - 1); i >= 0; i--)
#define VI vector<int>
#define VVI vector<vector<int>>
#define VVVI vector<vector<vector<int>>>
#define VL vector<ll>
#define VVL vector<vector<ll>>
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define PAIR pair<int,int>
#define MP make_pair
#define VP vector<pair<int,int>>
#define VS vector<string>
#define MAP map<int,int>
#define QUE queue<int>
#define DEQ deque<int>
#define PQUE priority_queue<int> //5,5,4,3,3,2,...
#define REPQUE priority_queue<int, vector<int>, greater<int>> //1,1,2,3,4,4,5,...
#define SUM(obj) accumulate((obj).begin(), (obj).end(), 0)
#define SORT(obj) sort((obj).begin(), (obj).end()) // 1,2,3,4,5...
#define RESORT(obj) sort((obj).begin(), (obj).end(), greater<int>()) // 5,4,3,2,1...
#define UB(obj,n) upper_bound((obj).begin(), (obj).end(), n) //itr > n
#define LB(obj,n) lower_bound((obj).begin(), (obj).end(), n) //itr>= n

const ll MOD = (ll)1e9 + 7;
const ll INF = (ll)1e17;

int gcd(int a, int b) {
	if (a == 0 || b == 0) return 0;
	if (a < b) swap(a, b);
	while (b != 0) {
		a = a%b;
		swap(a, b);
	}
	return a;
}

int lcm(int a, int b) {
	if (a == 0 || b == 0) return 0;
	return a / gcd(a, b)*b;
}

vector<bool> Eratosthenes(int N) {
	vector<bool> Eratosthenes(N + 1, true);
	Eratosthenes[0] = false;
	if (N == 0) return Eratosthenes;
	Eratosthenes[1] = false;
	for (int i = 1; i*i <= N; i++) if (Eratosthenes[i]) for (int j = 2 * i; j <= N; j += i) Eratosthenes[j] = false;
	return Eratosthenes;
}

bool pcheck(int N) {
	if (N == 0 || N == 1) return false;
	for (int i = 2; i*i <= N; i++) if (N % i == 0) return false;
	return true;
}

////combination mod-------------------------------------------------------------------------
//
//vector<ll> f;//erase this line when you use!
//
////vector including modulus of N!
//vector<ll> factorial(ll N, ll quotient) {
//	vector<ll> factorial(N + 1, 1);
//	for (ll i = 1; i <= N; i++) {
//		factorial[i] *=factorial[i - 1]*i;
//		factorial[i] %= quotient;
//	}
//	return factorial;
//}
//
////repeat square method y = x^n mod quotient 
//ll rsm(ll x,ll n,ll quotient){
//	ll y = 1;
//	while (n > 0) {
//		if ((n & 1) == 1) {
//			y *= x;
//			y %= quotient;
//		}
//		x *= x;
//		x %= quotient;
//		n >>= 1; 
//	}
//	return y;
//}
//
////modulus of nCk - combination mod 
//ll nCk(ll n, ll k, ll quotient) {
//	
//	ll nCk = (f[n] % quotient);
//	nCk *= rsm(f[k], quotient - 2, quotient);
//	nCk %= quotient;
//	nCk *= rsm(f[n - k], quotient - 2, quotient);
//	return (nCk % quotient);
//}
////------------------------------------------------------------

////union-find---------------------------------------------------
//int node[10];
//
//int root(int n) {
//	if (node[n] == n) return n;
//	else return node[n] = root(node[n]);
//}
//
//void unite(int n, int m) {
//	if (n > m) swap(n, m);
//	n = root(n);
//	m = root(m);
//	if (n == m) return;
//	else node[m] = n;
//}
//-------------------------------------------------------------

//void dijkstra(int start) {
//	priority_queue<PAIR, vector<PAIR>, greater<PAIR>> pq;
//	dist[start][start] = 0;
//	pq.push({ 0,start });
//	while (!pq.empty()) {
//		int from = pq.top().second;
//		pq.pop();
//		REP(i, edge[from].size()) {
//			int to = edge[from][i].first;
//			if (dist[start][to] > dist[start][from] + edge[from][i].second) {
//				dist[start][to] = dist[start][from] + edge[from][i].second;
//				pq.push({ dist[start][from] + edge[from][i].second,to });
//			}
//		}
//	}
//}


void ANS(bool flag) {
	cout << ((flag) ? "YES" : "NO") << endl;
}

void Ans(bool flag) {
	cout << ((flag) ? "Yes" : "No") << endl;
}

void ans(bool flag) {
	cout << ((flag) ? "yes" : "no") << endl;
}

int main() {
	string Sa, Sb, Sc;
	cin >> Sa >> Sb >> Sc;
	int a = 0,b = 0,c = 0,i = 0;

	while(1){
		if(i == 0){

			if (a == Sa.size()) {
				cout << "A" << endl;
				break;
			}

			i = int(Sa[a] - 'a');
			a++;
		}
		else if (i == 1) {

			if (b == Sb.size()) {
				cout << "B" << endl;
				break;
			}

			i = int(Sb[b] - 'a');
			b++;
		}
		else if (i == 2) {

			if (c == Sc.size()) {
				cout << "C" << endl;
				break;
			}

			i = int(Sc[c] - 'a');
			c++;
		}	
			
	}
	return 0;
}


Measured memory_bytes: 256