V7.III.5

#include<iostream>
#include<iomanip>
using namespace std;
int i,j,g;
int a[100],b[100],n,m,aux,OK;


int main()
{
cout<<"n="; cin>>n; m=n;
for(i=1;i<=n;i++)   {cout<<"a["<<i<<"]=";cin>>a[i];b[i]=a[i];}

for(i=1;i<=m;i++)
{
    OK=0;
    for(j=1;j<=m;j++)
    {
        if(b[i]==b[j])  OK++;
        if(OK>1)
        {
            for(g=j;g<m;g++)   b[g]=b[g+1];
            m--;    j--;    OK=1;
        }
    }
}

do{
OK=1;
for(i=1;i<=m-1;i++)
 {
     if(b[i]>b[i+1])
        {
            OK=0;
            aux=b[i];
            b[i]=b[i+1];
            b[i+1]=aux;
        }
 }
}while(OK!=1);

OK=1;
for(i=1;i<=m-2;i++)
{
    if(b[i+1]!=(b[i]+b[i+2])/2) OK=0;
}
if(OK)
{
    cout<<endl;
    int r=b[2]-b[1];
    cout<<r<<endl;
}
else
{
    cout<<"NU"<<endl;
}

cout<<endl;
return 1337;
}
// dragos sa-mi suga pula de penichez