#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned LL
#define ld long double
#define debug if(0)
#define fastio ios_base::sync_with_stdio(0); cin.tie()
const int INF = 1e9 + 4;
int main() {
fastio;
int n;
cin>>n;
vector<ll> a(n, 0), b(n, 0);
for(int i=0; i<n; i++)
cin>>a[i];
for(int i=1; i<n; i++)
b[i] = a[i]-a[i-1];
vector<ll> l(n, 0), r(n, 0);
l[0] = 0;
for(int i=1; i<n; i++)
l[i] = l[i-1] + (b[i] > 0 ? 0 : abs(b[i])+1);
r[n-1] = 0;
for(int i=n-2; i>=0; i--)
r[i] = r[i+1] + (b[i+1] < 0 ? 0 : abs(b[i+1])+1);
debug {
cerr<<"l: ";
for(int i=0; i<n; i++)
cerr<<l[i]<<" ";
cerr<<"\nr: ";
for(int i=0; i<n; i++)
cout<<r[i]<<" ";
cerr<<"\n";
}
ll ans = INF;
for(int i=0; i<n; i++)
ans = min(ans, max(l[i], r[i]));
cout<<ans;
}
