#include <iomanip.h>
#include <iostream.h>
int main()
{
    int n;
    float a,b,c,sum=0.0,c1=0.0;
    cin>>n;
    cin>>a>>b;
    for(int i=1;i<=n;i++)
    {
        cin>>c;
        c1+=c;
        sum+=c1;
    }
        cout.setf(ios::fixed);
        cout<<setprecision(2)<<(n*a+b-2*sum)/(n+1)<<endl;
}

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