#include <iostream>
#include <vector>
using namespace std;
using ll = long long;
template<class T> using vec = vector<T>;
template<class T> using vvec = vector<vec<T>>;

int main(){
    ll N;
    cin >> N;
    vector<ll> B(N);
    for(int i=0;i<N;i++) cin >> B[i];
    ll ans = 0;
    ll b = 0,c = 0;
    for(int i=0;i<N;i++){
        ans += b+c;
//        cerr << i << " " << b << " " << c << endl;
        if(B[i]==0){
            if(b>0){
                b--;
            }else c++;
        }else{
            if(c>0){
                if(c>B[i]-1){
                    c -= B[i]-1;
                }else{
                    b += B[i]-1-c;
                    c = 0;
                }
            }else{
                b += B[i]-1;
            }
        }
    }
    cout << ans << endl;
}