import java.io.PrintWriter;
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.LongStream;

public class Main {
    public static void main(String[] args) {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        Task task = new Task();
        task.solve(sc, out);
        out.flush();
        sc.close();
    }

    static class Task {
        public void solve(Scanner sc, PrintWriter out) {
            char[] sA = nstr(sc).toCharArray();
            char[] sB = nstr(sc).toCharArray();
            char[] sC = nstr(sc).toCharArray();

            char x = sA[0];
            int a = sA.length;
            int b = sB.length;
            int c = sC.length;
            a --;
            while(!(a==0 && x=='a') || (b==0 && x == 'b') || (c==0 && x=='c')){
                if (x == 'a'){
                    x = sA[sA.length - a];
                    a --;
                }else if(x == 'b'){
                    x = sB[sB.length - b];
                    b --;
                }else{
                    x = sC[sC.length - c];
                    c--;
                }
                if ((a==0 && x=='a') || (b==0 && x == 'b') || (c==0 && x=='c')){
                    break;
                }
            }
            out.println((a==0 && x=='a') ? "A" : (b==0 && x == 'b') ? "B" : "C");
        }
    }


    static int nint(Scanner sc) {


        return Integer.parseInt(sc.next());
    }

    static long nlong(Scanner sc) {
        return Long.parseLong(sc.next());
    }

    static double ndouble(Scanner sc) {
        return Double.parseDouble(sc.next());
    }

    static float nfloat(Scanner sc) {
        return Float.parseFloat(sc.next());
    }

    static String nstr(Scanner sc) {
        return sc.next();
    }

    static long[] longLine(Scanner sc, int size) {
        long[] lLine = new long[size];
        for (int i = 0; i < size; i++) {
            lLine[i] = nlong(sc);
        }
        return lLine;
    }

    static int[] intLine(Scanner sc, int size) {
        int[] iLine = new int[size];
        for (int i = 0; i < size; i++) {
            iLine[i] = nint(sc);
        }
        return iLine;
    }

    static String[] strLine(Scanner sc, int size) {
        String[] strLine = new String[size];
        for (int i = 0; i < size; i++) {
            strLine[i] = nstr(sc);
        }
        return strLine;
    }

    static long maxFromList(List<Long> longList) {
        return longList.stream().max(Comparator.naturalOrder()).get();
    }

    static long minFromList(List<Long> longList) {
        return longList.stream().min(Comparator.naturalOrder()).get();
    }

    public static int sumDigits(int n) {
        int sum = 0;
        while (n != 0) {
            sum += n % 10;
            n /= 10;
        }
        return sum;
    }

    public static long sumDigits(long n) {
        long sum = 0;
        while (n != 0) {
            sum += n % 10;
            n /= 10;
        }
        return sum;
    }

    static List<Integer> getIntegerList(Scanner sc, int size) {
        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < size; i++) {
            list.add(nint(sc));
        }
        return list;
    }

    static List<Long> getLongList(Scanner sc, int size) {
        List<Long> list = new ArrayList<>();
        for (int i = 0; i < size; i++) {
            list.add(nlong(sc));
        }
        return list;
    }

    private static long lcm(long a, long b) {
        return a * b / gcd(b, a % b);
    }

    private static long gcd(long a, long b) {
        return b == 0 ? a : gcd(b, a % b);
    }
}


Measured memory_bytes: 21332