import java.io.*;
import java.math.BigInteger;
import java.util.*;


public class B
{
        String line;
        StringTokenizer inputParser;
        BufferedReader is;
        FileInputStream fstream;
        DataInputStream in;
        String FInput="";
        
        void openInput(String file)
        {

                if(file==null)is = new BufferedReader(new InputStreamReader(System.in));//stdin
                else
                {
                        try{
                
                                
                        fstream = new FileInputStream(file);
                        in = new DataInputStream(fstream);
                        is = new BufferedReader(new InputStreamReader(in));
                        }catch(Exception e)
                        {
                                System.err.println(e);
                        }
                }

        }
        
        void readNextLine()
    	{
    		try {
    			line = is.readLine();
    			inputParser = new StringTokenizer(line, " ");
    			//System.err.println("Input: " + line);
    		} catch (IOException e) {
    			System.err.println("Unexpected IO ERROR: " + e);
    		}	
    		catch (NullPointerException e)
    		{
    			line=null;
    			
    		}
    		
    	}
        
        long NextLong()
        {
                String n = inputParser.nextToken();
                
                long val = Long.parseLong(n);
                
                 return val;
        }
        
        int NextInt()
        {
                String n = inputParser.nextToken();
                int val = Integer.parseInt(n);
                
                //System.out.println("I read this number: " + val);
                return val;
        }
        
        String NextString()
        {
                String n = inputParser.nextToken();
                return n;
        }
        
        void closeInput()
        {
                try {
                        is.close();
                } catch (IOException e) {
                        System.err.println("Unexpected IO ERROR: " + e);
                }
                        
        }
        
        
        public static void main(String [] argv)
        {
                String filePath=null;
                if(argv.length>0)filePath=argv[0];
                new B(filePath);
        }
        
        public void readFInput()
    	{
    		for(;;)
    		{
    			try
    			{
    				readNextLine();
    				FInput+=line+" ";
    			}
    			catch(Exception e)
    			{
    				break;
    			}
    		}
    		inputParser = new StringTokenizer(FInput, " ");
    	}
        
       
        public B(String inputFile)
        {
        	openInput(inputFile);
        	readNextLine();
    		
    		int k=NextInt();
    		int b=NextInt();
    		int n=NextInt();
    		int t=NextInt();

    		long x=1;
    		int i=0;
    		while(x<=t)
    		{
    			x=(x*k)+b;
    			i++;
    		}
    		System.out.println(Math.max(0, n-i+1));
    		
            closeInput();		
    	}

        private int gcd(int a, int b) {
    		if(b==0)return a;
    		return gcd(b, a%b);
    	}
	


       
}