void main()//2601
{
	int n,i;
	cin>>n;
	double a0,an,a1,ct=0.0;
	cin>>a0>>an;
	double *c=new double[n];
	for(i=0;i<n;i++)
	{
		cin>>c[i];
		ct=ct+c[i];
	}
	a1=a0*0.5+an*0.5-ct;
	printf("%.2d\n",a1);
}