#include <cstdlib>
#include <iostream>
#include <algorithm>
using namespace std;
int n,a[20002],cou,b;
int main()
{

    cin >> n;
    
    for(int i=0;i<n;i++)
    {
    cin >> a[i];
    }                 
//    sort(a,a+n);
    reverse( a, a+n);

    for(int i=0;i<n;i++)
    {
      if(a[i+1] != (a[i]-1)) 
      {
        do
        {
                if(a[i+1]> (a[i]-1))
                {
                           a[i+1] -= 1;
                           //cout << " -1 " << " " << a[i+1] << " " << endl;
                }           
                if(a[i+1]< (a[i]-1))
                {
                           a[i+1] += 1;
                          //cout << " +1 " << " " << a[i+1] << " " << endl;
                }           
                cou+=1;
                
        }
        while(a[i+1] != a[i]-1);  
      }     
    }
    b= (a[n]-1)+1;
    cout << cou-b;
    
    system("PAUSE");
    return EXIT_SUCCESS;
}