#include <bits/stdc++.h>
using namespace std;
using ll = long long;
struct Fenwick {
int n;
vector<ll> bit;
Fenwick(int _n) : n(_n), bit(n + 2, 0) {}
void add(int i, ll v) {
while (i <= n) {
bit[i] += v;
i += i & -i;
}
}
ll sum(int i) {
ll res = 0;
while (i > 0) {
res += bit[i];
i -= i & -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> comp = B;
sort(comp.begin(), comp.end());
comp.erase(unique(comp.begin(), comp.end()), comp.end());
int M = comp.size();
Fenwick fw(M + 2);
ll ans = 0;
for (int i = 0; i < N; ++i) {
int pos = lower_bound(comp.begin(), comp.end(), B[i]) - comp.begin() + 1;
ans += i - fw.sum(pos);
fw.add(pos, 1);
}
cout << ans << '\n';
return 0;
}
