Why does this code throw a runtime error?
Everything is right!!

from sys import stdin
pocht_1 = ["A", "P", "O", "R"]
pocht_2 = ["B", "M", "S"]
pocht_3 = ["D", "G", "J", "K", "T", "W"]
count = 0
mesto = 1
for i in sys.stdin.split("\n")[1:]:
    if i[0] in pocht_1:
        if mesto == 3:
            count += 2
        elif mesto == 2:
            count += 1
        mesto = 1
    elif i[0] in pocht_2:
        if mesto == 1:
            count += 1
        elif mesto == 3:
            count += 1
        mesto = 2
    else:
        if mesto == 1:
            count += 2
        elif mesto == 2:
            count += 1
        mesto = 3
print(count)