Buggy python submission (verdict: Runtime Error):
using System;
using static System.Console;
using System.Text;
using System.Linq;
using static System.Math;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;

class Program
{
    public static cp alg;
    public static int[] A;
    //public static List<int[]> B;
    static void Main(string[] args)
    {
        alg = new cp();
        A = alg.inputSplitInt();
        int n = A[0];
        int k = A[1];
        string s = alg.inputString();
        string ans = "";
        for(int a = 0; a < n; a++)
        {
            if (a == k-1)
            {
                ans += s[a].ToString().ToLower();
            }
            else
            {
                ans += s[a].ToString();
            }
        }
        WriteLine(ans);

    }
}

class cp
{

    public cp()
    {
    }
    public int GCD(int a, int b)
    {
        return a % b == 0 ? b : GCD(b, a % b);
    }
    public int LCM(int a, int b)
    {
        return a * b / GCD(a, b);
    }
    public int countString(string s, string key)
    {
        return (s.Length - s.Replace(key, "").Length) / key.Length;
    }
    public string Yn(bool f)
    {
        if (f)
        {
            return "Yes";
        }
        else
        {
            return "No";
        }
    }
    public string YN(bool f)
    {
        if (f)
        {
            return "YES";
        }
        else
        {
            return "NO";
        }
    }
    public string yn(bool f)
    {
        if (f)
        {
            return "yes";
        }
        else
        {
            return "no";
        }
    }
    public void inputSkip()
    {
        ReadLine();
        return;
    }
    public string inputString()
    {
        return ReadLine();
    }
    public string[] inputSplitString()
    {
        return ReadLine().Split();
    }
    public string[] inputLinesString(int len)
    {
        var arr = new string[len];
        for (int a = 0; a < len; a++)
        {
            arr[a] = ReadLine();
        }
        return arr;
    }
    public string[][] inputJaggedArrayString(int len)
    {
        var arr = new string[len][];
        for (int a = 0; a < len; a++)
        {
            arr[a] = ReadLine().Split();
        }
        return arr;
    }
    public int inputInt()
    {
        return int.Parse(ReadLine());
    }
    public int[] inputSplitInt()
    {
        return ReadLine().Split().Select(i => int.Parse(i)).ToArray();
    }
    public int[] inputLinesInt(int len)
    {
        var arr = new int[len];
        for (int a = 0; a < len; a++)
        {
            arr[a] = int.Parse(ReadLine());
        }
        return arr;
    }
    public int[][] inputJaggedArrayInt(int len)
    {
        var arr = new int[len][];
        for (int a = 0; a < len; a++)
        {
            arr[a] = ReadLine().Split().Select(i => int.Parse(i)).ToArray();
        }
        return arr;
    }
    public long inputLong()
    {
        return long.Parse(ReadLine());
    }
    public long[] inputSplitLong()
    {
        return ReadLine().Split().Select(i => long.Parse(i)).ToArray();
    }
    public long[] inputLinesLong(int len)
    {
        var arr = new long[len];
        for (int a = 0; a < len; a++)
        {
            arr[a] = long.Parse(ReadLine());
        }
        return arr;
    }
    public long[][] inputJaggedArrayLong(int len)
    {
        var arr = new long[len][];
        for (int a = 0; a < len; a++)
        {
            arr[a] = ReadLine().Split().Select(i => long.Parse(i)).ToArray();
        }
        return arr;
    }
    public double inputDouble()
    {
        return double.Parse(ReadLine());
    }
    public double[] inputSplitDouble()
    {
        return ReadLine().Split().Select(i => double.Parse(i)).ToArray();
    }
    public double[] inputLinesDouble(int len)
    {
        var arr = new double[len];
        for (int a = 0; a < len; a++)
        {
            arr[a] = double.Parse(ReadLine());
        }
        return arr;
    }
    public double[][] inputJaggedArrayDouble(int len)
    {
        var arr = new double[len][];
        for (int a = 0; a < len; a++)
        {
            arr[a] = ReadLine().Split().Select(i => double.Parse(i)).ToArray();
        }
        return arr;
    }
}

Fixed submission:
# coding: utf-8
import time
import os
import time
import re
s=input()
left=s[0:2]
right=s[2:4]
if left[0]==0:
    left=left[1]
if right[0]==0:
    right=right[1]
left=int(left)
right=int(right)
bo=[False for a in range(2)]#YYMM MMYY

#YYMM
if right>0 and right<13:
    bo[0]=True
#MMYY
if left>0 and left<13:
    bo[1]=True

if bo[0]==True and bo[1]==True:
    print("AMBIGUOUS")
elif bo[0]==False and bo[1]==False:
    print("NA")
elif bo[0]==True and bo[1]==False:
    print("YYMM")
else:
    print("MMYY")