#include <bits/stdc++.h>
using namespace std::literals::string_literals;
using i64 = std::int_fast64_t;
using std::cout;
using std::cerr;
using std::endl;
using std::cin;

template<typename T>
std::vector<T> make_v(size_t a){return std::vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return std::vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

int main() {
	int n; scanf("%d", &n); std::vector<int> b(n);
	for(int i = 0; i < n; i++) scanf("%d", &b[i]);

	i64 ans = 0;
	std::stack<int> L, R;
	for(int i = 0; i < n; i++) {
		if(b[i] == 1) continue;
		
		if(b[i]) {
			while(!L.empty() and b[i] > 1) {
				ans += (i - L.top());
				L.pop();
				b[i]--;
			}
			while(b[i]-- > 1) R.push(i);
		} else {
			if(R.empty()) L.push(i);
			else {
				ans += (i - R.top());
				R.pop();
			}
		}
	}

	printf("%lld\n", ans);
	return 0;
}