package main

import (
	"fmt"
)

func main() {
	var n, m int
	if _, err := fmt.Scan(&n, &m); err != nil {
		return
	}

	a := make([][]int, n+2)
	for i := range a {
		a[i] = make([]int, m+2)
	}

	for i := 1; i <= n; i++ {
		var s string
		fmt.Scan(&s)
		for j := 1; j <= m; j++ {
			if s[j-1] == 'W' {
				a[i][j] = 1
			} else if s[j-1] == 'B' {
				a[i][j] = -1
			}
		}
	}

	ans := 0
	for i := 1; i <= n; i++ {
		for j := 1; j <= m; j++ {
			val := a[i][j] - a[i+1][j] - a[i][j+1] + a[i+1][j+1]
			if val != 0 {
				ans++
			}
		}
	}
	fmt.Println(ans)
}