wrong ordering pairs

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

int a[500010], temp[500010];
void merge_sort(int, int);
int ans = 0;

int main(){
    int n;
    cin >> n;
    for(int i=0; i<n; i++) cin >> a[i];
    merge_sort(0, n);
    cout << ans << endl;
}

void merge_sort(int l, int r){
    if(l+1 == r) return;
    int mid = l + r >> 1;
    merge_sort(l, mid), merge_sort(mid, r);
    int lptr = l, rptr = mid, ptr = l;
    while(lptr < mid || rptr < r){
        if(lptr != mid && (rptr == r || a[lptr] < a[rptr]))
            temp[ptr++] = a[lptr++], ans += rptr - mid;
        else temp[ptr++] = a[rptr++];
    }
    for(int i=l; i<r; i++) a[i] = temp[i];
}