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;
}516. Longest Palindromic Subsequence | 468. Validate IP Address |
450. Delete Node in a BST | 445. Add Two Numbers II |
442. Find All Duplicates in an Array | 437. Path Sum III |
436. Find Right Interval | 435. Non-overlapping Intervals |
406. Queue Reconstruction by Height | 380. Insert Delete GetRandom O(1) |
332. Reconstruct Itinerary | 368. Largest Divisible Subset |
377. Combination Sum IV | 322. Coin Change |
307. Range Sum Query - Mutable | 287. Find the Duplicate Number |
279. Perfect Squares | 275. H-Index II |
274. H-Index | 260. Single Number III |
240. Search a 2D Matrix II | 238. Product of Array Except Self |
229. Majority Element II | 222. Count Complete Tree Nodes |
215. Kth Largest Element in an Array | 198. House Robber |
153. Find Minimum in Rotated Sorted Array | 150. Evaluate Reverse Polish Notation |
144. Binary Tree Preorder Traversal | 137. Single Number II |