#include <cstdio>
using namespace std;

int a[103];

int main (){
    int n;
    scanf ("%d",&n);
    for (int i=0;i<n;++i){
        scanf ("%d",&a[i]);
    }
    int zbr=0;
    for (int i=n-2;i>=0;--i){
        if (a[i+1]-a[i]<=0){
           zbr+=a[i]-a[i+1]+1;
           a[i]=a[i]-(a[i]-a[i+1]+1);
        }
    }
    printf ("%d\n",zbr);
    return 0;
}