#include <bits/stdc++.h>
#define ff first
#define ss second
using namespace std;
using ll = long long;
int n;
vector<ll> a, pref, suff;
int main(){
cin >> n;
a.resize(n);
pref.resize(n, 0);
suff.resize(n, 0);
for(int i = 0; i < n; i++) cin >> a[i];
for(int i = 1; i < n; i++) if(a[i - 1] >= a[i]) pref[i] = a[i - 1] - a[i] + 1;
for(int i = n - 2; i >= 0; i--) if(a[i] <= a[i + 1]) suff[i] = a[i + 1] - a[i] + 1;
//for(int i = 0; i < n; i++) cout << pref[i] << ' '; cout << '\n';
//for(int i = 0; i < n; i++) cout << suff[i] << ' '; cout << "\n\n";
int l = 0, r = n - 1;
ll lsum = 0, rsum = 0;
while(l <= r){
if(lsum < rsum){
lsum += pref[l];
l++;
}else{
rsum += suff[r];
r--;
}
}
cout << max(lsum, rsum);
}
