#include <cstdio>

using namespace std;

int N[200];
int main()
{
    int n;
    scanf("%d",&n);
    for (int i=0; i<n; i++)
        scanf("%d", &(N[i]));

    int sol = 0;
    for (int i=n-2; i>=0; i--) {
        if (N[i+1] <= N[i]) {
            int t = N[i] - N[i+1] + 1;
            N[i] -= t;
            sol+=t;
        }
    }
    printf("%d\n", sol);
}