Buggy python submission (verdict: Runtime Error):
Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
K = N-1    
allw = False
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(K, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
                K = j
#                 print(Cc)
                break
        else:
            allw = True

    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
    if allw:
        break

Fixed submission:
import sys
import numpy as np
import math
import collections
import copy
from collections import deque 
from functools import reduce
from itertools import product
from itertools import combinations

# input = sys.stdin.readline
N = int(input())
Cn = list(input())

Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
K = N-1    
allw = False
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(K, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
                K = j
#                 print(Cc)
                break
        else:
            allw = True

    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
    if allw:
        break

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
import numpy as np
import math
import collections
import copy
from collections import deque 
from functools import reduce
from itertools import product
from itertools import combinations

# input = sys.stdin.readline
N = int(input())
Cn = list(input())
Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
K = N-1    
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(K, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
                K = j
#                 print(Cc)
                break
    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
print(ans)


















Fixed submission:
import sys
import numpy as np
import math
import collections
import copy
from collections import deque 
from functools import reduce
from itertools import product
from itertools import combinations

# input = sys.stdin.readline
N = int(input())
Cn = list(input())

Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
K = N-1    
allw = False
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(K, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
                K = j
#                 print(Cc)
                break
        else:
            allw = True

    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
    if allw:
        break

print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
import numpy as np
import math
import collections
import copy
from collections import deque 
from functools import reduce
from itertools import product
from itertools import combinations

# input = sys.stdin.readline
N = int(input())
Cn = list(input())
Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
    
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(N-1, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
#                 print(Cc)
                break
    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
print(ans)


















Fixed submission:
import sys
import numpy as np
import math
import collections
import copy
from collections import deque 
from functools import reduce
from itertools import product
from itertools import combinations

# input = sys.stdin.readline
N = int(input())
Cn = list(input())

Cc = copy.copy(Cn)
if (Cn[0] == "R"):
    Rflg = True
else:
    Rflg = False
ans = 0
K = N-1    
allw = False
for i in range(N):
    if Rflg and Cc[i] == "R":
        continue
    else:
        Rflg = True
    if Cc[i] == "W":
        for j in range(K, i, -1):
            if Cc[j] == "R":
#                 print("change")
                ans += 1
                Cc[i], Cc[j] = Cc[j], Cc[i]
                K = j
#                 print(Cc)
                break
        else:
            allw = True

    else:
#         print("replace")
        ans += 1
        Cc[i] = "W"
#         print(Cc)
    if allw:
        break

print(ans)