import java.io.*; import java.util.*; public class Main { public static void main(String args[]) throws IOException { Reader.init(System.in); int n = Reader.nextInt(); String [] pho = new String [n]; for (int i = 0; i < pho.length; i++) { pho[i]= Reader.reader.readLine(); }boolean ok = false; int c =0; for (int i = 0; i < pho[0].length(); i++) { for (int j = 1; j <pho.length; j++) { if(pho[0].charAt(i)!=pho[j].charAt(i)){ System.out.println(c);return; } }c++; } } } class Reader { static BufferedReader reader; static StringTokenizer tokenizer; static void init(InputStream input) { reader = new BufferedReader( new InputStreamReader(input)); tokenizer = new StringTokenizer(""); } static String next() throws IOException { while (!tokenizer.hasMoreTokens()) { tokenizer = new StringTokenizer( reader.readLine()); } return tokenizer.nextToken(); } static int nextInt() throws IOException { return Integer.parseInt(next()); } static double nextDouble() throws IOException { return Double.parseDouble(next()); } }