#include <bits/stdc++.h>
using namespace std;
#define int long long
int n,m;
string a[20];
signed main(){
    cin>>n>>m;
    for(int i=1;i<=n;i++){
        cin>>a[i];
    }
    do{
        int cnt;
        for(int i=1;i<n;i++){
            cnt=0;
            for(int j=0;j<m;j++){
                cnt+=(a[i][j]!=a[i+1][j]);
            }
            if(cnt!=1) break;
        }
        if(cnt==1){
            cout<<"Yes";
            return 0;
        }
    }while(next_permutation(a+1,a+n+1));
    cout<<"No";
    return 0;
}

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