730 B | 730 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
def solve():
w, h = mii()
l = [input() for _ in range(h)]
ans = 0
dy = [0,0,1,-1]
dx = [1,-1,0,0]
for i in range(h):
for j in range(w):
for d in range(4):
ny = i + dy[d]
nx = j + dx[d]
if ny < 0 or nx < 0 or ny >= h or nx >= w:
continue
if l[i][j] != l[ny][nx]:
ans += 1
print(ans // 2)
if __name__ == "__main__":
tc = 1
for t in range(tc):
solve()