#include <cstdlib>
#include <iostream>

using namespace std;

int main () {
    int a,n,r,z;
    int niz[110];
    cin >> n;
    for (int i=0;i<n;i++)
    {
        cin >> niz[i];
    }
    z=niz[n-1];
    r=0;
    for (int i=(n-2);i>-1;i--)
    {
        if (niz[i]>=z)
        {
                      a=niz[i]-z;
                      niz[i]-=a+1;
                      r+=a+1;
        }
        z=niz[i];
    }
    cout << r << endl;
    return 0;
}