#include <bits/stdc++.h>
using namespace std;
#define int long long
void solve() {
int n;cin>>n;
vector<int>arr(n+1),pref(n+1),suff(n+2);
for(int i=1;i<=n;i++)cin>>arr[i];
pref[0]=0;pref[1]=0;
for(int i=2;i<=n;i++){pref[i]=pref[i-1];if(arr[i]<arr[i-1]+1) pref[i]+=(arr[i-1]-arr[i]+1);}
suff[n]=0;suff[n+1]=0;
for(int i=n-1;i>=1;i--){suff[i]=suff[i+1];if(arr[i]<arr[i+1]+1)suff[i]+=(arr[i+1]-arr[i]+1);}
int ans=LLONG_MAX;
for(int i=1;i<=n;i++)ans=min(ans,max(pref[i],suff[i]));
cout<<ans<<'\n';
}
signed main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int t=1;//in>>t;
while(t--)solve();
}
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