Petya loves lucky numbers very much. Everybody knows that lucky numbers are positive integers whose decimal record contains only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Petya has two strings a and b of the same length n. The strings consist only of lucky digits. Petya can perform operations of two types: 

  * replace any one digit from string a by its opposite (i.e., replace 4 by 7 and 7 by 4); 
  * swap any pair of digits in string a. 



Petya is interested in the minimum number of operations that are needed to make string a equal to string b. Help him with the task.

Input

The first and the second line contains strings a and b, correspondingly. Strings a and b have equal lengths and contain only lucky digits. The strings are not empty, their length does not exceed 105.

Output

Print on the single line the single number — the minimum number of operations needed to convert string a into string b.

Examples

Input

47
74


Output

1


Input

774
744


Output

1


Input

777
444


Output

3

Note

In the first sample it is enough simply to swap the first and the second digit.

In the second sample we should replace the second digit with its opposite.

In the third number we should replace all three digits with their opposites.
import java.awt.*;
import java.io.*;
import java.math.*;
import java.util.*;
import static java.lang.Math.*;

public class CF104_Div1_A implements Runnable {

	BufferedReader in;
	PrintWriter out;
	StringTokenizer tok = new StringTokenizer("");

	public static void main(String[] args) {
		new Thread(null, new CF104_Div1_A(), "", 256 * (1L << 20)).start();
	}

	public void run() {
		try {
			long t1 = System.currentTimeMillis();
			if (System.getProperty("ONLINE_JUDGE") != null) {
				in = new BufferedReader(new InputStreamReader(System.in));
				out = new PrintWriter(System.out);
			} else {
				in = new BufferedReader(new FileReader("input.txt"));
				out = new PrintWriter("output.txt");
			}
			Locale.setDefault(Locale.US);
			solve();
			in.close();
			out.close();
			long t2 = System.currentTimeMillis();
			System.err.println("Time = " + (t2 - t1));
		} catch (Throwable t) {
			t.printStackTrace(System.err);
			System.exit(-1);
		}
	}

	String readString() throws IOException {
		while (!tok.hasMoreTokens()) {
			tok = new StringTokenizer(in.readLine());
		}
		return tok.nextToken();
	}

	int readInt() throws IOException {
		return Integer.parseInt(readString());
	}

	long readLong() throws IOException {
		return Long.parseLong(readString());
	}

	double readDouble() throws IOException {
		return Double.parseDouble(readString());
	}
	
	BigInteger readBigInteger() throws IOException {
		return new BigInteger(readString());
	}
	
	void debug(Object... objects) {
		System.err.println(Arrays.deepToString(objects));
	}

	static class Utils {

		private Utils() {}
		
		
		private static final int MAGIC_VALUE = 50;
		

		public static void mergeSort(int[] a) {
			mergeSort(a, 0, a.length - 1);
		}
		public static void mergeSort(long[] a) {
			mergeSort(a, 0, a.length - 1);
		}
		public static void mergeSort(double[] a) {
			mergeSort(a, 0, a.length - 1);
		}


		private static void mergeSort(int[] a, int leftIndex, int rightIndex) {
			if (leftIndex < rightIndex) {
				if (rightIndex - leftIndex <= MAGIC_VALUE) {
					insertionSort(a, leftIndex, rightIndex);
				} else {
					int middleIndex = (leftIndex + rightIndex) / 2;
					mergeSort(a, leftIndex, middleIndex);
					mergeSort(a, middleIndex + 1, rightIndex);
					merge(a, leftIndex, middleIndex, rightIndex);
				}
			}
		}
		private static void mergeSort(long[] a, int leftIndex, int rightIndex) {
			if (leftIndex < rightIndex) {
				if (rightIndex - leftIndex <= MAGIC_VALUE) {
					insertionSort(a, leftIndex, rightIndex);
				} else {
					int middleIndex = (leftIndex + rightIndex) / 2;
					mergeSort(a, leftIndex, middleIndex);
					mergeSort(a, middleIndex + 1, rightIndex);
					merge(a, leftIndex, middleIndex, rightIndex);
				}
			}
		}
		private static void mergeSort(double[] a, int leftIndex, int rightIndex) {
			if (leftIndex < rightIndex) {
				if (rightIndex - leftIndex <= MAGIC_VALUE) {
					insertionSort(a, leftIndex, rightIndex);
				} else {
					int middleIndex = (leftIndex + rightIndex) / 2;
					mergeSort(a, leftIndex, middleIndex);
					mergeSort(a, middleIndex + 1, rightIndex);
					merge(a, leftIndex, middleIndex, rightIndex);
				}
			}
		}

		
		private static void merge(int[] a, int leftIndex, int middleIndex, int rightIndex) {
			int length1 = middleIndex - leftIndex + 1;
			int length2 = rightIndex - middleIndex;
			int[] leftArray = new int[length1];
			int[] rightArray = new int[length2];
			System.arraycopy(a, leftIndex, leftArray, 0, length1);
			System.arraycopy(a, middleIndex + 1, rightArray, 0, length2);
			for (int k = leftIndex, i = 0, j = 0; k <= rightIndex; k++) {
				if (i == length1) {
					a[k] = rightArray[j++];
				} else if (j == length2) {
					a[k] = leftArray[i++];
				} else {
					a[k] = leftArray[i] <= rightArray[j] ? leftArray[i++] : rightArray[j++];
				}
			}
		}
		private static void merge(long[] a, int leftIndex, int middleIndex, int rightIndex) {
			int length1 = middleIndex - leftIndex + 1;
			int length2 = rightIndex - middleIndex;
			long[] leftArray = new long[length1];
			long[] rightArray = new long[length2];
			System.arraycopy(a, leftIndex, leftArray, 0, length1);
			System.arraycopy(a, middleIndex + 1, rightArray, 0, length2);
			for (int k = leftIndex, i = 0, j = 0; k <= rightIndex; k++) {
				if (i == length1) {
					a[k] = rightArray[j++];
				} else if (j == length2) {
					a[k] = leftArray[i++];
				} else {
					a[k] = leftArray[i] <= rightArray[j] ? leftArray[i++] : rightArray[j++];
				}
			}
		}
		private static void merge(double[] a, int leftIndex, int middleIndex, int rightIndex) {
			int length1 = middleIndex - leftIndex + 1;
			int length2 = rightIndex - middleIndex;
			double[] leftArray = new double[length1];
			double[] rightArray = new double[length2];
			System.arraycopy(a, leftIndex, leftArray, 0, length1);
			System.arraycopy(a, middleIndex + 1, rightArray, 0, length2);
			for (int k = leftIndex, i = 0, j = 0; k <= rightIndex; k++) {
				if (i == length1) {
					a[k] = rightArray[j++];
				} else if (j == length2) {
					a[k] = leftArray[i++];
				} else {
					a[k] = leftArray[i] <= rightArray[j] ? leftArray[i++] : rightArray[j++];
				}
			}
		}

		
		private static void insertionSort(int[] a, int leftIndex, int rightIndex) {
			for (int i = leftIndex + 1; i <= rightIndex; i++) {
				int current = a[i];
				int j = i - 1;
				while (j >= leftIndex && a[j] > current) {
					a[j + 1] = a[j];
					j--;
				}
				a[j + 1] = current;
			}
		}
		private static void insertionSort(long[] a, int leftIndex, int rightIndex) {
			for (int i = leftIndex + 1; i <= rightIndex; i++) {
				long current = a[i];
				int j = i - 1;
				while (j >= leftIndex && a[j] > current) {
					a[j + 1] = a[j];
					j--;
				}
				a[j + 1] = current;
			}
		}
		private static void insertionSort(double[] a, int leftIndex, int rightIndex) {
			for (int i = leftIndex + 1; i <= rightIndex; i++) {
				double current = a[i];
				int j = i - 1;
				while (j >= leftIndex && a[j] > current) {
					a[j + 1] = a[j];
					j--;
				}
				a[j + 1] = current;
			}
		}

	}
	
	boolean isLucky(long x) {
		if (x == 0) return false;
		while (x > 0) {
			if (x % 10 != 4 && x % 10 != 7) return false;
			x /= 10;
		}
		return true;
	}
	
	long[] generateLuckyNumbers(long limit) {
		ArrayList<Long> list = new ArrayList<Long>();
		int maxLen = Long.toString(limit).length();
		for (int len = 1; len <= maxLen; len++) {
			for (int mask = 0; mask < (1 << len); mask++) {
				long num = 0;
				for (int i = 0; i < len; i++) {
					if (((1 << i) & mask) == 0) {
						num = num * 10 + 4;
					} else {
						num = num * 10 + 7;
					}
				}
				if (num <= limit) {
					list.add(num);
				}
			}
		}
		long[] res = new long[list.size()];
		for (int i = 0; i < res.length; i++) {
			res[i] = list.get(i);
		}
		Utils.mergeSort(res);
		return res;
	}

	// solution

	void solve() throws IOException {
		char[] a = readString().toCharArray();
		char[] b = readString().toCharArray();
		int n = a.length;
		int a47 = 0, a74 = 0;
		for (int i = 0; i < n; i++) {
			if (a[i] == '4' && b[i] == '7') {
				a47++;
			}
			if (a[i] == '7' && b[i] == '4') {
				a74++;
			}
		}
		int ans = max(a47, a74);
		out.println(ans);
	}

}
