tkSFgN - Online C++0x Compiler & Debugging Tool

#include<bits/stdc++.h>
using namespace std;
const long long MaxN = 1e5 + 5;
long long n,a[MaxN],res=0;
map<long long, long long> mp;
void input()
{
    cin >> n;
    for (long long i=1; i<=n; i++)
    {
        cin >> a[i];
    }
}
void solve()
{
    for (long long i=1; i<=n; i++)
    {
        mp[a[i]^i]++;
    }
    for (auto x: mp)
    {
        res = res + x.second*(x.second-1)/2;
    }
    cout << res;
}
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    input();
    solve();
}