#include <iostream>
#include <cstdlib>

using namespace std;
int n,v[110],mx,d,i,k;

int main ()
{
    cin>>n;
    for (i=0;i<n;i++) cin>>v[i];
    mx=v[n-1];
    for (i=n-2;i>=0;i--)
    {  
        if (mx>v[i]) mx=v[i];
        else { k=v[i]; mx=v[i]-(v[i]-mx+1); d=d+k-mx;} 
    }
    
    cout<<d;
    
//system("PAUSE");
return 0;
}