Progresie3

#include <fstream>
#define N 1001
using namespace std;
ifstream f("progresie3.in");
ofstream g("progresie3.out");
int F[N];

int main()
{
	int a;
	while(f>>a)
		F[a]=1;
    int r=0,ultim=-1;
    bool ok=true;
    for(int i=0;i<N&&ok;++i)
		if(F[i])
        {
			if(ultim!=-1)
			{
				if(r==0)
					r=i-ultim;
                else
                    if(i-ultim!=r)
                        ok=false;
			}
			ultim=i;
		}
	if(ok)
		g<<r;
	else
		g<<"NU";
	return 0;
}