#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define ff first
#define ss second
#define all(s) s.begin(),s.end()
#define rall(s) s.rbegin(),s.rend()
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
ll n;
cin>>n;
ll a[n+5];
for(ll i=1;i<=n;i++){
cin>>a[i];
}
ll pre[n+5],suf[n+5];
pre[1]=0;
suf[n]=0;
for(ll i=2;i<=n;i++){
pre[i]=pre[i-1];
if(a[i-1]>=a[i]){
pre[i]+=a[i-1]-a[i]+1;
}
}
for(ll i=n-1;i>=1;i--){
suf[i]=suf[i+1];
if(a[i+1]>=a[i]){
suf[i]+=a[i+1]-a[i]+1;
}
}
ll ans=1e18;
for(ll i=1;i<=n;i++){
ans=min(ans,max(pre[i],suf[i]));
}
cout<<ans;
}
