#include<bits/stdc++.h>
#define ll long long
#define pii pair<ll,ll>
#define f first
#define s second
#define pb push_back
#define all(x) x.begin(),x.end()
using namespace std;
const int N=1e5+5,B=1000;
int main() {
ios_base::sync_with_stdio(0);cin.tie(0);
int n;cin>>n;
int a[n+1];
for(int i=1;i<=n;i++)cin>>a[i];
int dpl[n+1];dpl[1]=0;
for(int i=2;i<=n;i++)dpl[i]=dpl[i-1]+(a[i-1]-a[i]>=0?a[i-1]-a[i]+1:0);
int dpr[n+1];dpr[n]=0;
for(int i=n-1;i>=1;i--)dpr[i]=dpr[i+1]+(a[i+1]-a[i]>=0?a[i+1]-a[i]+1:0);
int ans=2e9;
for(int i=1;i<=n-1;i++)ans=min(ans,max(dpl[i],dpr[i]));
cout<<ans;
}
