#include <iostream>

using namespace std;
const int maxn=110;
int n,a[maxn],maks,z;

int main (void){
    cin >> n;
    for (int i=0;i<n;i++){
            cin >> a[n];
    }
    maks=a[n-1];
    maks--;
    for (int i=n-2;i>=0;i--){
            z+=a[i]-maks;
            maks--;
    }
    cout << z << endl;
    return 0;
}