#include<iostream>
using namespace std;
int main(){
	long long n,a=0,ans=0;
	cin>>n;
	for(int i=1;i<=n;i++){
		int b;
		cin>>b;
		if(a>b){
			ans+=a+1;
			a+=1;
			
		}
		else  {
			ans+=b;
			a=b;
		}
	}
cout << ans;	
}