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 2
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 5
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 1
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import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.InputMismatchException;

public class NewProblem {
    public static void main(String[] args) {
        FasterScanner sc = new FasterScanner();
        
        int N = sc.nextInt();

        HashSet<String> used = new HashSet<String>();
        for (int i = 0; i < N; i++) {
        	String X = sc.nextLine();
        	int len = X.length();
        	for (int j = 0; j < len; j++) {
        		int idx2 = Math.min(j + 2, len);
        		int idx3 = Math.min(j + 3, len);
        		used.add(X.substring(j, j + 1));
        		used.add(X.substring(j, idx2));
        		used.add(X.substring(j, idx3));
        	}
        }
        
        for (int i = 1; ; i++) {
        	int[][] all = getAll(26, i);
        	for (int[] arr : all) {
        		String str = makeStr(arr);
        		if (!used.contains(str)) {
        			System.out.println(str);
        			return;
        		}
        	}
        }
    }
    
    public static String makeStr(int[] arr) {
    	StringBuilder sb = new StringBuilder();
    	for (int n : arr) {
    		sb.append((char)(n + 'a'));
    	}
    	return sb.toString();
    }

    /**
     * Method for computing permutations with repeats.  Returns an array of int[], which contains k elements from { 0, 1, ..., n - 1 }.
     * Each of the int[] elements will be sorted in ascending order.
     */
    public static int[][] getAll(int n, int k)
    {
        ArrayList<int[]> lst = new ArrayList<int[]>();
        int[] arr = new int[k];
        allHelper(n, k, 0, arr, lst);
        return lst.toArray(new int[0][]);
    }

    /**
     * Helper method for computing permutations with repeats.  Used by GetAll(...).
     */
    private static void allHelper(int n, int k, int t, int[] curr, ArrayList<int[]> lst)
    {
        if (t == k)
        {
            int[] copy = new int[k];
            System.arraycopy(curr, 0, copy, 0, k);
            lst.add(copy);
            return;
        }

        for (int i = 0; i < n; i++)
        {
            curr[t] = i;
            allHelper(n, k, t + 1, curr, lst);
        }
    }
    
	public static class FasterScanner {
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public int read() {
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) {
				curChar = 0;
				try {
					numChars = System.in.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

		public String nextLine() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}

		public String nextString() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		public long nextLong() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			long res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public int nextInt() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') {
				sgn = -1;
				c = read();
			}
			int res = 0;
			do {
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
	        
	    public int[] nextIntArray(int n) {
	        return nextIntArray(n, 0);
	    }
	    
	    public int[] nextIntArray(int n, int off) {
	    	int[] arr = new int[n + off];
	    	for (int i = 0; i < n; i++) {
	    		arr[i + off] = nextInt();
	    	}
	    	return arr;
	    }
	    
	    public long[] nextLongArray(int n) {
	    	return nextLongArray(n, 0);
	    }
        
		public long[] nextLongArray(int n, int off) {
		    long[] arr = new long[n + off];
		    for (int i = 0; i < n; i++) {
		        arr[i + off] = nextLong();
		    }
		    return arr;
		}

	    private boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
	}
}