763 B | 763 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
n = ii()
l = [[*map(int, list(input()))] for _ in range(n)]
cnt = 0
for i in range(n - 1, -1, -1):
for j in range(n - 1, -1, -1):
if l[i][j] == 1:
cnt += 1
for y in range(i + 1):
for x in range(j + 1):
l[y][x] = 1 - l[y][x]
p(cnt)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()