#include <stdio.h>
#include <string.h>

main() {

int i=0,isrectangle=1;
char s[80];
int row,col;

fgets(s,78,stdin);
sscanf(s,"%d%d",&row,&col);

while (fgets(s,78,stdin) !=NULL) {
	i++;
	if((strlen(s)-1) != col) isrectangle=0;
	}
if(row != i) isrectangle=0;

if(isrectangle) printf("YES\n"); else printf("NO\n");

}