#include <cstdio>

const int MAXN=100;
int n, p[MAXN], rj;
int f(int);
void print();

int main(){
	scanf ("%d", &n);
	for (int i=0; i<n; ++i)
		scanf ("%d", &p[i]);
	for (int i=n-1; i>0; --i){
		rj+=f(p[i-1]-p[i]);
		p[i-1]=p[i]-1;
	}
	printf ("%d", rj);
	//scanf ("%*c%*c");
	return 0;
}
int f(int a){
	if (a<0) return 0;
	return a+1;
}
void print(){
	for (int i=0; i<n; ++i)
		printf ("%d ", p[i]);
	printf ("\n\n");
}