851 B | 851 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
# sys.setrecursionlimit(10**7)
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline().rstrip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def gcd(a, b): return gcd(b, a % b) if b > 0 else a
def lcm(a, b): return a * b // gcd(a, b)
def solve():
n = ii()
l = mii()
for i in range(n):
v = l[i]
left_dis = n - i
l[i] = min(v, left_dis)
prev = l[-1]
ans = prev
for i in range(n - 2, -1, -1):
crt = l[i]
if crt <= prev:
ans += crt
prev = crt
else:
ans += prev + 1
prev = prev + 1
p(ans)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()