465B - Inbox (100500) - CodeForces Solution

#include <bits/stdc++.h>
#define deb(x) cout<<endl<<#x <<" = "<<x<<endl<<endl
using namespace std;
typedef long long ll;
typedef double dd;

int main()
{
    ll tt; tt=1;
    while(tt--)
    {
        ll n; cin>>n;
        ll a[n];
        for(ll i=0;i<n;i++) cin>>a[i];

        for(ll i=1;i<n-1;i++) if(a[i-1]==1 && a[i+1]==1) a[i]=1;

        ll ans=0,buff=0;

        for(ll i=0;i<n;i++){
            if(a[i]){
                buff++;
            }
            else if(buff){
                ans+=buff;
                ans++;
                buff=0;
            }
        }
        if(buff){
            ans+=buff;
            ans++;
        }
        cout<<max(0ll,--ans)<<endl;
    }
}