#include <fstream>
#include <bitset>
using namespace std;
ifstream fin("memory004.in");
ofstream fout("memory004.out");

struct secv { unsigned short s=0,d=0;};
bitset<10001> prim; // prim[i] == 1 - neprim / prim[i] == 0 - prim

secv P[1002],M[1002],U[1002],C[1002];


int main()
{
    unsigned short n,m,a;
    fin>>n>>m;
    prim[0] =prim[1] = 1;
    for (int i = 4; i <= 10000; i += 2)
    prim[i] = 1;
    for (int i = 3; i * i <= 10000; i += 2) {
        if (prim[i] == 0) {
            for (int j = i * i; j <= 10000; j += i)
                prim[j] = 1;
        }
    }
    for(int i=1;i<=n;i++)
        for(int j=1;j<=m;j++)
        {
            fin>>a;
            if(prim[a]==0)
            {
                if(C[j].s==0)
                    C[j].s=C[j].d=i;
                else C[j].d=i;
                if(C[j].d-C[j].s>=M[j].d-M[j].s)
                    M[j]=C[j];
                if(C[j].s==1) P[j]=C[j];
                if(C[j].d==n) U[j]=C[j];
            }
            else C[j].s=C[j].d=0;
        }

    int l=0,lmax=0;
    for(int j=1;j<=m;j++)
    {
        if(P[j].s==1 && (j==1 || U[j-1].d==n))
        {   l=l+P[j].d-P[j].s+1;
            if(l>lmax) lmax=l;
        }
        else l=P[j].d-P[j].s+1;
        if(U[j].d!=n) l=0;
        else if(U[j].s!=1) l=U[j].d-U[j].s+1;
        if(M[j].s!=1 && M[j].d!=n)
            if(M[j].d-M[j].s+1>lmax) lmax=M[j].d-M[j].s+1;
    }
    fout<<lmax;
    return 0;
}