#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
void speed()
{
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
}
int main()
{
speed();
int n; cin>>n;
ll a[n];
for(int i=0; i<n; i++)cin>>a[i];
ll razlike[n]; razlike[0]=0; //razlike[i]=a[i]-a[i-1]
for(int i=1; i<n; i++)razlike[i]=a[i]-a[i-1];
ll preff[n]={0}, suff[n]={0};
preff[0]=0; suff[n-1]=0;
for(int i=1; i<n; i++)
{
if(razlike[i]<=0)preff[i]=-razlike[i]+1;
preff[i]+=preff[i-1];
}
for(int i=n-2; i>=0; i--)
{
if(razlike[i+1]>=0)suff[i]=razlike[i+1]+1;
suff[i]+=suff[i+1];
}
//for(int i=0; i<n; i++)cout<<suff[i]<<" "<<preff[i]<<" "<<razlike[i]<<endl;
ll rezz=INT_MAX;
for(int i=0; i<n; i++)
{
rezz=min(rezz, max(suff[i], preff[i]));
}
cout<<rezz;
return 0;
}
