#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define MAXN (1000005)
int main() {
ios_base::sync_with_stdio(false);cin.tie(0);
ll N;
cin>>N;
ll A[N], arr[N];
for(ll i = 0;i < N;i++){
cin>>arr[i];
A[i] = arr[i];
}
ll prefix[N];
ll cur = 0, add = 0;
for(ll i = 0;i < N;i++){
arr[i] += add;
if(i >= 1){
cur += max(0ll,(arr[i - 1] + 1) - arr[i]);
add += max(0ll,(arr[i - 1] + 1) - arr[i]);
arr[i] = max(arr[i],arr[i - 1] + 1);
}
prefix[i] = cur;
}
for(ll i = 0;i < N;i++){ //reset
arr[i] = A[i];
}
ll suffix[N];
cur = 0;
add = 0;
for(ll i = N - 1;i >= 0;i--){
arr[i] += add;
if(i < N - 1){
cur += max(0ll,(arr[i + 1] + 1) - arr[i]);
add += max(0ll,(arr[i + 1] + 1) - arr[i]);
arr[i] = max(arr[i],arr[i + 1] + 1);
}
suffix[i] = cur;
}
for(ll i = 0;i < N;i++){ //reset
arr[i] = A[i];
}
ll minimum = 1e18;
minimum = min(minimum,suffix[0]);
for(ll split = 0;split < N - 1;split++){
ll extra = max(0ll,(A[split] + prefix[split] + 1) - (A[split+1] + suffix[split+1]));
minimum = min(minimum,prefix[split] + extra + suffix[split + 1]);
}
minimum = min(minimum,prefix[N - 1]);
cout<<minimum<<'\n';
}
