	import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;
	
	
	public class CF {
		public static void main(String[] args) {
			FasterScanner sc = new FasterScanner();
			PrintWriter out = new PrintWriter(System.out);
			long K = sc.nextLong();
			long B = sc.nextLong();
			long N = sc.nextLong();
			long T = sc.nextLong();
		
			long total = 1;
			long many = 0;
			while(total <= T){
				total = total*K + B;
				many++;
			}
			
			out.println(Math.max(0, N-many+1));
			out.close();
		}
		

	
		static class FasterScanner {
			private InputStream stream;
			private byte[] buf = new byte[1024];
			private int curChar;
			private int numChars;
	
			public FasterScanner() {
				stream = System.in;
				// stream = new FileInputStream(new File("dec.in"));
	
			}
	
			int read() {
				if (numChars == -1)
					throw new InputMismatchException();
				if (curChar >= numChars) {
					curChar = 0;
					try {
						numChars = stream.read(buf);
					} catch (IOException e) {
						throw new InputMismatchException();
					}
					if (numChars <= 0)
						return -1;
				}
				return buf[curChar++];
			}
	
			boolean isSpaceChar(int c) {
				return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
			}
	
			boolean isEndline(int c) {
				return c == '\n' || c == '\r' || c == -1;
			}
	
			int nextInt() {
				return Integer.parseInt(next());
			}
	
			long nextLong() {
				return Long.parseLong(next());
			}
	
			double nextDouble() {
				return Double.parseDouble(next());
			}
	
			String next() {
				int c = read();
				while (isSpaceChar(c))
					c = read();
				StringBuilder res = new StringBuilder();
				do {
					res.appendCodePoint(c);
					c = read();
				} while (!isSpaceChar(c));
				return res.toString();
			}
	
			String nextLine() {
				int c = read();
				while (isEndline(c))
					c = read();
				StringBuilder res = new StringBuilder();
				do {
					res.appendCodePoint(c);
					c = read();
				} while (!isEndline(c));
				return res.toString();
			}
	
		}
	
	}