代码

#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include<cstdio>
#include<algorithm>
using namespace std;
using namespace __gnu_pbds;
tree<int , null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> t;
int n,p;
long long ans;
int main(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        scanf("%d",&p);
        ans+=i-t.order_of_key(p)-1;
        t.insert(p);
    }
    printf("%lld",ans);
}

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