import sys
input = lambda: sys.stdin.readline().rstrip()

def main():
    # 入力
    N, M = map(int, input().split())
    D = [sorted(list(map(int, input().split()))) for _ in range(N)]
    # 計算・出力        
    ng = -1
    ok = 10**9
    while abs(ng-ok) > 1:
        mid = (ok+ng)//2

        dp = [True]*M
        for i in range(1, N):
            dp0 = [False]*M
            l = 0
            r = 0
            bool = 0
            for j in range(M):
                while r < M and D[i-1][r] <= D[i][j]:
                    bool += 1 if dp[r] else 0
                    r += 1
                while l < M and D[i-1][l] < D[i][j] - mid:
                    bool -= 1 if dp[l] else 0
                    l += 1
                dp0[j] = (bool > 0)
            dp = dp0
        
        if any(dp) : # ここ頑張る
            ok = mid
        else:
            ng = mid
    print(ok if ok != 10**9 else -1)

if __name__ == "__main__":
    main()