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";
}479C - Exams | 1030A - In Search of an Easy Problem |
158A - Next Round | 71A - Way Too Long Words |
160A - Twins | 1A - Theatre Square |
1614B - Divan and a New Project | 791A - Bear and Big Brother |
1452A - Robot Program | 344A - Magnets |
96A - Football | 702B - Powers of Two |
1036A - Function Height | 443A - Anton and Letters |
1478B - Nezzar and Lucky Number | 228A - Is your horseshoe on the other hoof |
122A - Lucky Division | 1611C - Polycarp Recovers the Permutation |
432A - Choosing Teams | 758A - Holiday Of Equality |
1650C - Weight of the System of Nested Segments | 1097A - Gennady and a Card Game |
248A - Cupboards | 1641A - Great Sequence |
1537A - Arithmetic Array | 1370A - Maximum GCD |
149A - Business trip | 34A - Reconnaissance 2 |
59A - Word | 462B - Appleman and Card Game |