package main

import (
	"fmt"
)

func main() {
	var n int
	fmt.Scan(&n)
	var s string
	fmt.Scan(&s)
	for i := 0; i < n-1; i++ {
		if s[i] != s[i+1] {
			fmt.Println("YES")
			fmt.Println(s[i : i+2])
			return
		}
	}
	fmt.Println("NO")
}