import java.io.*;
import java.util.*;
import java.util.stream.*;

public class Main {
    static int[][] matrix;
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int m = sc.nextInt();
        matrix = new int[n][m];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                matrix[i][j] = sc.nextInt();
            }
            Arrays.sort(matrix[i]);
        }
        int left = -1;
        int right = Integer.MAX_VALUE / 2;
        while (right - left > 1) {
            int mm = (left + right) / 2;
            if (check(mm, n - 1)) {
                right = mm;
            } else {
                left = mm;
            }
        }
        if (right >= 1000000000) {
            System.out.println(-1);
        } else {
            System.out.println(right);
        }
    }
    
    static boolean check(int x, int idx) {
        boolean ans = false;
        for (int y : matrix[idx]) {
            if (check(idx - 1, y, x)) {
                return true;
            }
        }
        return false;
    }
    
    static boolean check(int idx, int x, int diff) {
        if (idx < 0) {
            return true;
        }
        for (int y : matrix[idx]) {
            if (y > x) {
                return false;
            }
            if (y + diff >= x) {
                if (check(idx -1, y, diff)) {
                    return true;
                }
            }
        }
        return false;
    }
}
class Utilities {
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}