#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(a); (i)<(b); ++(i))
#define rFor(i, a, b) for(int (i)=(a)-1; (i)>=(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef pair<lint, lint> pll;
typedef complex<double> xy_t;
template<class T>bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T>bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}

constexpr lint mod = 1e9+7;
constexpr lint INF = mod*mod;
constexpr int MAX = 100010;

int main(){
    int n;
    scanf("%d", &n);
    lint b[n], s[n+1];
    s[0]=0;
    rep(i, n){
        scanf("%lld", &b[i]);
        s[i+1]=s[i]+b[i];
    }
    lint ans=0;
    rep(i, n){
        ans+=max(0LL, s[i]-i);
        ans+=max(0LL, s[n]-s[i+1]-(n-1-i));
    }
    printf("%lld\n", ans);
}