#include <iostream>

using namespace std;

int main(){
    int n,a[100],suma=0;
    cin >>n;
    for (int i=0; i<n; i++){
        cin >>a[i];
        }
    for (int i=n-1; i>=0; i--){
        if (i!=n-1 & a[i]>=a[i+1])
        suma+=(a[i]-a[i+1]+1),
        a[i]=a[i+1]-1;
        }
        cout <<suma;
    //system ("pause");
    return 0;
    }