#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("bac.in");


int main()
{
    int n,F[1001]={0},a=-1,r=0,ok=1;
    while(fin>>n)
        F[n]=1; //vector caracteristic
    for(int i=0;i<=1000;i++)
        if(F[i]==1)//element care apare
        {
            if(r==0)
                { //calculez ratia din primele 2
                    if(a!=-1) r=i-a;
                }
            else if(i-a!=r) ok=0; //verific daca ratia se pastreza
            a=i; //retin elementul anterior
        }
    if(ok) cout<<r;
    else cout<<"NU";

}