#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
#define F first
#define S second
#define FOR(i,a,b) for (auto i = (a); i <= (b); ++i)
#define NFOR(i,a,b) for(auto i = (a); i >= (b); --i)
#define all(x) (x).begin(), (x).end()
#define sz(x) lli(x.size())
#define mp(i,a) make_pair(i,a)
#define pb(a) push_back(a)
#define bit(x,b) (x&(1LL<<b))
typedef long long int lli;
typedef pair <lli,lli> ii;
typedef pair <ii,lli> iii;
typedef vector <lli> vi;
typedef tree<lli,null_type,less<lli>,rb_tree_tag,tree_order_statistics_node_update> ordered_set;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
lli n;
cin>>n;
lli a[n],b[n]={},e[n]={};
for(lli i=0;i<n;++i)
{
cin>>a[i];
if(i==0)
b[0]=0;
else
b[i]=max(a[i-1]+1-a[i],0*1LL)+b[i-1];
}
for(lli i=n-2;i>=0;--i)
e[i]=e[i+1]+max(0*1LL,a[i+1]+1-a[i]);
lli ans=e[0]+b[0];
for(lli i=0;i<n;++i)
ans=min(ans,max(b[i],e[i]));
cout<<ans<<endl;
return 0;
}
