#include <cstdlib>
#include <iostream>

using namespace std;

int razina[101];

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