n, m = (int(i) for i in input().split())
prev, res = "", "YES"
for r in range(n):
s = input()
if not all(c == s[0] for c in s) or s[0] == prev:
res = "NO"
break
prev = s[0]
print(res)
#include <iostream>
using namespace std;
int main()
{
int n,m;cin>>n>>m;
char x,y;
for(int i=0;i<n;i++)
{
int a[10]={0};
for(int j=0;j<m;j++)
{
cin>>x;
a[x-48]++;
}
if(a[x-48]!=m)
{
cout<<"NO";return 0;
}
else
{
if(i==0)
y=x;
else
{
if(y==x)
{
cout<<"NO";return 0;
}
else
y=x;
}
}
}
cout<<"YES";
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