int main(){
    char s[502];
    int max,frequency[251],n,i,j;
    scanf("%d",&n);
    scanf("%s",s);
    max=1;
    if(n==2){
        for(i=0;i<strlen(s)-2;i++){
            frequency[i]=1;
            for(j=i+1;j<strlen(s)-1;j++){
                if(s[i]==s[j]&&s[i+1]==s[j+1])
                    frequency[i]++;
            }
            if(max<frequency[i])
                max=frequency[i];
        }
        if(max==1)
            printf("NO\n");
        else{
            printf("%d\n",max);
            for(i=0;i<strlen(s)-2;i++){
                if(frequency[i]==max)
                    printf("%c%c\n",s[i],s[i+1]);
            }
        }
    }
    if(n==3){
        for(i=0;i<strlen(s)-3;i++){
            frequency[i]=1;
            for(j=i+1;j<strlen(s)-2;j++){
                if(s[i]==s[j]&&s[i+1]==s[j+1]&&s[i+2]==s[j+2])
                    frequency[i]++;
            }
            if(max<frequency[i])
                max=frequency[i];
        }
        if(max==1)
            printf("NO\n");
        else{
            printf("%d\n",max);
            for(i=0;i<strlen(s)-3;i++){
                if(frequency[i]==max)
                    printf("%c%c%c\n",s[i],s[i+1],s[i+2]);
            }
        }
    }
    if(n==4){
        for(i=0;i<strlen(s)-4;i++){
            frequency[i]=1;
            for(j=i+1;j<strlen(s)-3;j++){
                if(s[i]==s[j]&&s[i+1]==s[j+1]&&s[i+2]==s[j+2]&&s[i+3]==s[j+3])
                    frequency[i]++;
            }
            if(max<frequency[i])
                max=frequency[i];
        }
        if(max==1)
            printf("NO\n");
        else{
            printf("%d\n",max);
            for(i=0;i<strlen(s)-4;i++){
                if(frequency[i]==max)
                    printf("%c%c%c%c\n",s[i],s[i+1],s[i+2],s[i+3]);
            }
        }
    }
    return 0;
    }