#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;
using mint = modint998244353;

int main() {
  ll N; cin >> N;
  vector<ll> A(N);
  for (auto &x: A) cin >> x;
  ll cnt = 1, ans = 0;
  for (ll i = 0; i < N - 1; i++) {
    if (A[i] + 1 == A[i + 1]) {
      cnt++;
      ans += cnt - 1;
    } else {
      cnt = 1;
    }
  }
  cout << ans << endl;
  return 0;
}