#include<iostream>
#include<vector>
#include<algorithm>
#include<set>
#include<map>
#include<queue>
#include<cmath>
#include<iomanip>
#include<cstring>
#define initdp(a,b) for(int i=0;i<=a;i++)for(int j=0;j<=b;j++)dp[i][j]=-1;
#define fi first
#define se second
#define pb push_back
#define pii pair<int,int>
#define ll long long
#define pll pair<ll,ll>
#define rep(i,n) for(int i=0;i<n;i++)
#define repd(i,n) for(int i=n-1;i>=0;i--)
#define inf 1000000001
#define inf1 1000000000000000001
#define mod 1000000007
#define pie 3.14159265358979323846
#define N 1000005
#define mid(l,r) l+(r-l)/2
using namespace std;
int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
int ddx[8]={1,1,0,-1,-1,-1,0,1},ddy[8]={0,1,1,1,0,-1,-1,-1};
void mad(ll &a,ll b){a=(a+b)%mod;if(a<0)a+=mod;}
ll gcd(ll a,ll b){ if(a>b)swap(a,b);if(!a)return b;return gcd(b%a,a);}

void solve(){
	ll int n;
	cin>>n;
	ll zc=0,ex=0,ans=0;
	ll a[n];
	rep(i,n)cin>>a[i];
	rep(i,n){
		ans+=zc+ex;
		if(a[i]>1){
			ll d=1;
			if(zc)d=max(1ll,a[i]-zc);
			d=a[i]-d;
			if(d>zc){ex+=d-zc;zc=0;}
			else zc-=d;
		}
		else if(!a[i]){
			if(ex)ex--;
			else zc++;
		}
	}
	cout<<ans+zc+ex;
}
int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	cout.tie(NULL);
	int t;
	//cin>>t;
	t=1;
	while(t--){
		solve();
	}
}