cat chars.py
```
f = open("flag.txt", "r")
flag = ""

lines = f.readlines()

for i in range(0,len(lines)):
line = lines[i]
flag += line[i]
print flag
```

python chars.py

cmFkYXJ7Y2hhcl9hZnRlcl9jaGFyX2dpdmVfZmxhZ30

b64decode(cmFkYXJ7Y2hhcl9hZnRlcl9jaGFyX2dpdmVfZmxhZ30) = radar{char_after_char_give_flag}