#include<iostream>
#include<cstdio>

using namespace std;

int n,br;
int main(){
 cin>>n;
 int niz[n];
 for(int i=0;i<n;i++)cin>>niz[i];
 for(int i=n-2;i>=0;i--){
         if(niz[i]>niz[i+1]){
                             br=br+(niz[i]-niz[i+1])+1;
                             niz[i]=niz[i+1]-1;
                             }
         if(niz[i]==niz[i+1]){
                              niz[i]=niz[i+1]-1;
                              br=br+1;                           
                              }
         }
 cout<<br<<endl;
 //system("pause");
 return 0;
}