Buggy python submission (verdict: Runtime Error):
s = input()
n = len(s)

if n < 2:
    print(0)

else:
    ans, i = 0, 0
    rem = []

    while i < n:

        if not rem:

            if i+1 < n:
                if s[i] == s[i+1]:
                    rem.append(s[i])
                    i += 1

                else:
                    i += 2
                    ans += 2

            else:
                i += 1

        else:
            if rem[-1] != s[i]:
                rem.pop()
                i += 1
                ans += 2
            
            else:
                if i + 1 < n:
                    if s[i] == s[i+1]:
                        rem.append(s[i])
                        i += 1

                    else:
                        i += 2
                        ans += 2

                else:
                    i += 1
print(ans)


Fixed submission:
#%%
s = input()
n = len(s)

if n < 2:
    print(0)

else:
    ans, i = 0, 0
    rem = []

    while i < n:

        if not rem:

            if i+1 < n:
                if s[i] == s[i+1]:
                    rem.append(s[i])
                    i += 1

                else:
                    i += 2
                    ans += 2

            else:
                i += 1

        else:
            if rem[-1] != s[i]:
                rem.pop()
                i += 1
                ans += 2
            
            else:
                if i + 1 < n:
                    if s[i] == s[i+1]:
                        rem.append(s[i])
                        i += 1

                    else:
                        i += 2
                        ans += 2

                else:
                    i += 1
    print(ans)

Buggy python submission (verdict: Runtime Error):
#%%
def c(s):
    n = len(s)

    if n < 2:
        print(0)

    else:
        ans, i = 0, 0
        rem = []

        while i < n:
            print(rem,s[0:i],s[i:])

            if not rem:

                if i+1 < n:
                    if s[i] == s[i+1]:
                        rem.append(s[i])
                        i += 1

                    else:
                        i += 2
                        ans += 2

                else:
                    i += 1

            else:
                if rem[-1] != s[i]:
                    rem.pop()
                    i += 1
                    ans += 2
                
                else:
                    if i + 1 < n:
                        if s[i] == s[i+1]:
                            rem.append(s[i])
                            i += 1

                        else:
                            i += 2
                            ans += 2

                    else:
                        i += 1
    print(ans)

#%%
s = input()

c(s)

Fixed submission:
#%%
s = input()
n = len(s)

if n < 2:
    print(0)

else:
    ans, i = 0, 0
    rem = []

    while i < n:

        if not rem:

            if i+1 < n:
                if s[i] == s[i+1]:
                    rem.append(s[i])
                    i += 1

                else:
                    i += 2
                    ans += 2

            else:
                i += 1

        else:
            if rem[-1] != s[i]:
                rem.pop()
                i += 1
                ans += 2
            
            else:
                if i + 1 < n:
                    if s[i] == s[i+1]:
                        rem.append(s[i])
                        i += 1

                    else:
                        i += 2
                        ans += 2

                else:
                    i += 1
    print(ans)