#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i,n) for(int i = 0;i < (int)(n);i++)
#define RREP(i,n) for(int i = (int)n-1;i >= 0;i--)
#define FOR(i,s,n) for(int i = s;i < (int)n;i++)
#define RFOR(i,s,n) for(int i = (int)n-1;i >= s;i--)
#define ALL(a) a.begin(),a.end()
#define IN(a, x, b) (a<=x && x<b)
template<class T>inline bool CHMAX(T&a,T b){if(a<b){a = b;return true;}return false;}
template<class T>inline bool CHMIN(T&a,T b){if(a>b){a = b;return true;}return false;}
constexpr long long INF = 1e18;


signed main(){
	int N;
	cin>>N;
	vector<int>a(N);
	REP(i,N){
		cin>>a[i];
	}
	int husoku = 0,sum = 0;
	int yobun = 0;
	int husokusum=0,yobunsum=0;
	REP(i,N){
		if(a[i]==0&&yobun==0){
			husoku++;
		}else if(a[i]==0&&yobun){
			yobun--;
		}else{
			yobun+=a[i]-1;
			while(husoku&&yobun){
				husoku--;
				yobun--;
			}
		}
		husokusum+=husoku;
		yobunsum+=yobun;
	}
	cout<<husokusum+yobunsum<<endl;
}