#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ int s1=1,m; char s[1001]; cin>>m>>s; for(int i=1;s[i]!='\0';++i){ if(s[i]==s[i+1]){ if(s[i+1]==s[i+2]){ cout<<s[i]; break; } } else{ s1=1; } } if(s1!=1){ cout<<"No"; } return 0; }