import java.io.IOException;
import java.io.InputStream;
import java.util.*;

public class Main {
    public static void main(String[] args){
        FastScanner sc = new FastScanner();
        String A = sc.next();
        String B = sc.next();
        String C = sc.next();
        int counterA = 0;
        int counterB = 0;
        int counterC = 0;
        Character winner = 'A';
        Character subtractedCharacter = 'a';
        outer:
        while (true) {

            switch (subtractedCharacter){
                case 'a':
                    if(counterA == A.length()){
                        winner = 'A';
                        break outer;
                    }
                    subtractedCharacter = A.charAt(counterA);
                    counterA++;
                    break;
                case 'b':
                    if(counterB == B.length()){
                        winner = 'B';
                        break outer;
                    }
                    subtractedCharacter = B.charAt(counterB);
                    counterB++;
                    break ;
                case 'c':
                    if(counterC == C.length()){
                        winner = 'C';
                        break outer;
                    }
                    subtractedCharacter = C.charAt(counterC);
                    counterC++;
                    break;
            }


        }
        System.out.println(winner);




    }

    static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;
        private boolean hasNextByte() {
            if (ptr < buflen) {
                return true;
            }else{
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0) {
                    return false;
                }
            }
            return true;
        }
        private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
        private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
        public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
        public String next() {
            if (!hasNext()) throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while(isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }
        public long nextLong() {
            if (!hasNext()) throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while(true){
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                }else if(b == -1 || !isPrintableChar(b)){
                    return minus ? -n : n;
                }else{
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }
        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
            return (int) nl;
        }
        public double nextDouble() { return Double.parseDouble(next());}
    }
}


Measured memory_bytes: 21204