#include <iostream>
#include <cmath>
#include <string>
#include <cstdio>
#include <algorithm>
using namespace std;
int c[110];
int main()
{
    int N,d=0;
    scanf("%d", &N);
    for(int i=0;i<N;i++)scanf("%d", &c[i]);
    for(int i=N-1;i>0;i--)
    {
            if(c[i]<=c[i-1]) {d+=c[i-1]-c[i]+1; c[i-1]-=c[i-1]-c[i]+1;
            }
            }
            printf("%d\n", d);
return 0;
}