x=int(input())
s=list(map(int,input().split()))[::-1]
for n in range(1,x):
s[n]=max(0,min(s[n-1]-1,s[n]))
print(sum(s))
#include <bits/stdc++.h>
using namespace std;
# define ll long long int
int main() {
ll n;
cin>>n;
ll arr[n];
for(int i=0;i<n;i++) {
cin>>arr[i];
}
ll count=arr[n-1];
ll k=arr[n-1];
for(int i=n-2;i>=0;i--) {
if(k-1>=0){
ll j=min(k-1,arr[i]);
count+=j;
k=j;
}
}
cout<<count;
return 0;
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