749 B | 749 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
# sys.setrecursionlimit(10**7)
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline().rstrip()
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():
k = ii()
a = inp()
b = inp()
n = len(a)
same = 0
diff = 0
for i in range(n):
if a[i] == b[i]:
same += 1
else:
diff += 1
same_ans = min(k, same)
k -= same_ans
diff_ans = diff - k
p(same_ans + diff_ans)
if __name__ == "__main__":
tc = ii()
for t in range(1, tc+1):
ret = solve()