#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define dbg if(0)
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin>>n;
vector<ll> a(n), b(n), l(n, 0), r(n, 0);
for(int i=0; i<n; i++)
cin>>a[i];
for(int i=0; i<(n-1); i++)
b[i] = a[i+1] - a[i];
for(int i=1; i<n; i++)
l[i] = l[i-1] + max(0LL, 1-b[i-1]);
for(int i=(n-2); i>=0; i--)
r[i] = r[i+1] + max(0LL, b[i]+1);
dbg {
cerr<<"l: ";
for(int i=0; i<n; i++)
cerr<<l[i]<<" ";
cerr<<"\nr: ";
for(int i=0; i<n; i++)
cerr<<r[i]<<" ";
cerr<<"\n";
}
ll ans = max(l[0], r[0]);
for(int i=2; i<n; i++)
ans = min(ans, max(l[i], r[i]));
cout<<ans;
}
