// 還要更強
#include<iostream>
#include<queue>
#include<set>
#include<map>
#include<iomanip>
#include<math.h>
#include<cstring>
#include<stack>
#include<string.h>
#include<random>
#include<algorithm>
#include<vector>
#define ll long long
#define FOR(i,a,b) for(int i=a;i<b;i++)
#define REP(i,a,b) for(int i=a;i>=b;i--)
#define INF (ll)(2e18)
#define pb push_back
#define AC ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
using namespace std;
ll b[200005],a[200005];
vector<ll> r,l;
signed main(){
AC;
bool ok=1;
int n; cin>>n;
FOR(i,1,n+1) cin>>a[i];
FOR(i,1,n+1){
b[i]=a[i]-a[i-1];
if(b[i]>=0) r.pb(i);
else l.pb(i);
if(b[i]==0) ok=0;
}
if(ok&&r[r.size()-1]<l[0]){
cout<<0<<endl; return 0;
}
reverse(l.begin(),l.end());
ll ans=0;
FOR(i,2,n+1){
if(b[i]==0){
ans++;
if(r.size()&&l.size()&&r.back()>l.back()){
if(b[r.back()]==0){
r.pop_back();
}
b[r.back()]--;
}
}
else if(b[i]<0&&r.size()&&r.back()>i){
l.pop_back();
ans+=(1-b[i]);
if(r.size()&&l.size()&&l.back()<r.back()){
b[r.back()]-=(1-b[i]);
if(b[r.back()]<0){
r.pop_back();
}
}
}
//cout<<"i="<<i<<" ans="<<ans<<endl;
}
cout<<ans<<endl;
}
