#include <bits/stdc++.h>
using namespace std;
using ll = long long;
struct Fenwick {
int n;
vector<ll> f;
Fenwick(int _n): n(_n), f(n+2, 0) {}
void update(int i, ll v) {
for (; i <= n; i += i & -i) f[i] += v;
}
ll query(int i) {
ll res = 0;
for (; i > 0; i -= i & -i) res += f[i];
return res;
}
};
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
vector<ll> A(N), B(N);
for (int i = 0; i < N; ++i) {
cin >> A[i];
B[i] = A[i] + 2LL * i;
}
// Coordinate compression
vector<ll> vals = B;
sort(vals.begin(), vals.end());
vals.erase(unique(vals.begin(), vals.end()), vals.end());
int M = vals.size();
Fenwick bit(M + 2);
ll ans = 0;
for (int i = 0; i < N; ++i) {
int pos = lower_bound(vals.begin(), vals.end(), B[i]) - vals.begin() + 1;
ans += i - bit.query(pos);
bit.update(pos, 1);
}
cout << ans ;
return 0;
}
