#include<iostream>

using namespace std;
int n=0, niz[100], x=0;
int main(void)
{
    
    
    
    cin>>n;
    for (int i = 0 ; i < n ; i ++ )
        cin>>niz[i];
        
    for (int i = n-1 ; i > 0 ; i -- )
        {
        
             if (niz[i-1]>=niz[i]) 
                {
                  x+=(niz[i-1]-niz[i])+1;
                  niz[i-1]-= (niz[i-1]-niz[i])+1;                                  
                }    
        }    
    cout<<x;
   
    return 0;    
}