```
#import pandas as pd
msg = ''
with open('msg.txt', 'rb') as f:
msg += f.read().decode()

#print(pd.Series(list(msg)).value_counts())
c_tmp = 'a'
count = 0
flag = ""
for c in msg + " ":
if c != c_tmp:
c_tmp = c
flag += chr(count)
count = 1
else:
count += 1

print(flag)
```

Flag: `bcactf{ch4r4ct3r_fr3qu3ncy_15_50_c00l_55aFejnb}`