#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define fi first
#define se second

const int maxn = 2e5;

int n;
int v[1 + maxn];
vector<int> to_upd[1 + maxn];

int tree[1 + 4 * maxn];

void update(int v, int vl, int vr, int pos, int val) {
    if(vl == vr) {
        tree[v] += val;
    } else {
        int mid = (vl + vr) / 2;
        if(pos <= mid) {
            update(2 * v, vl, mid, pos, val);
        } else {
            update(2 * v + 1, mid + 1, vr, pos, val);
        }
        tree[v] = tree[2 * v] + tree[2 * v + 1];
    }
}

int query(int v, int vl, int vr, int ql, int qr) {
    if(ql > vr || qr < vl) {
        return 0;
    }
    if(vl == ql && vr == qr) {
        return tree[v];
    }
    int mid = (vl + vr) / 2;
    return query(2 * v, vl, mid, ql, min(qr, mid))
        + query(2 * v + 1, mid + 1, vr, max(ql, mid + 1), qr);
}

void solve() {
    cin >> n;
    for(int i = 1; i <= n; i++) {
        cin >> v[i];
    }
    long long ans = 0;
    for(int i = n; i >= 1; i--) {
        ans += query(1, 1, n, 1, min(n, i + v[i]));
        update(1, 1, n, i, 1);
        to_upd[max(1, i - v[i])].pb(i);
        for(int x : to_upd[i]) {
            update(1, 1, n, x, -1);
        }
    }
    cout << ans << "\n";
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    solve();
}