#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;++i)
#define all(x) (x).begin(),(x).end()
using namespace std;
const int INF=1145141919,MOD=1e9+7;
const int64_t LINF=8931145141919364364,LMOD=998244353;
// const int dy[]={0,-1,0,1,1,-1,-1,1};
// const int dx[]={1,0,-1,0,1,1,-1,-1};

int main() {
	int n; cin >> n;
	int64_t empty = 0;
	int64_t wait = 0;
	int64_t ans = 0;
	for(int i = 0; i < n; ++i) {
		int a; cin >> a;
		if(a == 0) {
			if(wait > 0) {
				wait--;
			} else {
				empty++;
			}
		} else if(a != 1) {
			a--;
			wait += a;
			if(wait >= empty) {
				wait -= empty;
				empty = 0;
			} else {
				empty -= wait;
				wait = 0;
			}
		}
		ans += (wait + empty);
	}
	cout << ans << endl;
	return 0;
}