shopping

#include <iostream>
#include<iomanip>
using namespace std;
double v[1001],s[1001];
int n;

double s2(int i1)
{
    int i2=i1+1;
    if(v[i1]>v[i2])
        return v[i1]+v[i2]/2.;
    return v[i2]+v[i1]/2.;
}

double s3(int i1)
{
    double a=v[i1],b=v[i1+1],c=v[i1+2];
    if(a>b) swap(a,b);
    if(c<a) swap(a,c);
    return b+c;
}

int main()
{
    cin>>n;
    for(int i=1;i<=n;++i) cin>>v[i];
    s[1]=v[1];
    s[2]=min(s[1]+v[2],s2(1));
    for(int i=3;i<=n;++i)
        s[i]=min(s[i-1]+v[i],min(s[i-2]+s2(i-1),s[i-3]+s3(i-2)));
    cout<<fixed<<setprecision(1)<<s[n];
    return 0;
}