C++ :

#include<iostream>
using namespace std;
const int N = 3e5+5;
int n;
int a[N], b[N];
long long ans = 0;
void merge(int l, int r){
	int mid = l + r >> 1;
	if(l>=r)
		return;
	merge(l, mid);
	merge(mid+1, r);
	int i = l, j = mid + 1, k = 0;
	while(i<=mid && j<=r)	{
		if(a[i]>a[j])		{
			ans += mid - i + 1;
			b[++k] = a[j++];
		}
		else
			b[++k] = a[i++];
	}
	while(i<=mid)
		b[++k] = a[i++];
	while(j<=r)
		b[++k] = a[j++];
	
	for(int i=l;i<=r;i++)
		a[i] = b[i-l+1];
}
int main(){
	cin>>n;
	for(int i=n;i;i--)
		cin>>a[i];
	merge(1, n);
	cout<<ans;
	return 0;
}