#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3,unroll-loops")
using namespace std;
#define int long long
#define pii pair<int,int>
#define ff first
#define ss second
#define sp << " " <<
#define all(cont) cont.begin(),cont.end()
#define vi vector<int>
const int inf = 1e17,N = 3e5+1,MOD = 1e9+7,BL = 1000;
void solve() {
int n;
cin >> n;
vi a(n+1);
for (int i=1;i<=n;i++) cin >> a[i];
vi b(n+1,0),c(n+2,0);
vi x(n+1,0),y(n+2,0);
for (int i=1;i<=n;i++) {
b[i] = max(a[i],b[i-1]+1);
x[i] = x[i-1]+(b[i]-a[i]);
}
for (int i=n;i>=1;i--) {
c[i] = max(a[i],c[i+1]+1);
y[i] = y[i+1]+(c[i]-a[i]);
}
int ans = inf;
vi mni(n+1);
for (int i=1;i<=n;i++) {
int lift = max({(a[i-1]+x[i-1])+1,(a[i+1]+y[i+1])+1,a[i]})-a[i];
int sol = x[i-1],sag = y[i+1];
int anss = max({lift,sol,sag});
ans = min(ans,anss);
}
cout << ans << '\n';
}
int32_t main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#ifdef Dodi
freopen("in.txt","r",stdin);
freopen("out.txt","w",stdout);
#endif
int t = 1;
//cin >> t;
while (t --> 0) solve();
}
