#include <bits/stdc++.h>
#define N 200005
#define ff first
#define ss second
typedef long long ll;
const ll MOD = 1000000007;
const ll INF = 1000000000;
using namespace std;
ll n, a[N], l[N], r[N], suml[N], sumr[N], mxl[N], mxr[N], befl[N], befr[N];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
for(ll i = 1; i <= n; i++){
cin >> a[i];
}
ll L = 0, R = n + 1;
for(ll i = 1; i <= n; i++){
l[i] = max(l[i - 1] + 1, a[i]);
if(l[i] - a[i] == 0){
mxl[i] = 0;
suml[i] = suml[i - 1] + mxl[i - 1];
L = i;
}
else{
befl[i] = L;
mxl[i] = max(l[i] - a[i], mxl[i - 1]);
suml[i] = suml[i - 1];
}
}
for(ll i = n; i >= 1; i--){
r[i] = max(r[i + 1] + 1, a[i]);
if(r[i] - a[i] == 0){
mxr[i] = 0;
sumr[i] = sumr[i + 1] + mxr[i + 1];
R = i;
}
else{
befr[i] = R;
mxr[i] = max(r[i] - a[i], mxr[i + 1]);
sumr[i] = sumr[i + 1];
}
}
ll ans = 1e18;
for(ll i = 1; i <= n; i++){
ans = min(ans, suml[befl[i]] + sumr[befr[i]] + max(max(mxl[i - 1], mxr[i + 1]), max(l[i] - a[i], r[i] - a[i])));
}
cout << ans;
}
