#include<iostream>

using namespace std;

int main()
{
    int n,razina[105]={0},rj=0;
    cin>>n;
    for(int i=0; i<n; ++i)
    {
        cin>>razina[i];
    }
    for(int i=n-1; i>0; --i)
    {
        if(razina[i]<=razina[i-1])
        {
            rj+=razina[i-1]-razina[i]+1;
            razina[i-1]-=(razina[i-1]-razina[i])+1;
        }
    }
    cout<<rj;
    return 0;
}