炸了，求调，悬关

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

typedef long long l;
int a[500005], b[500005];
l ans;

#define r register

char inbuf[1 << 20], *st(inbuf), *en(inbuf), outbuf[1 << 20], *outptr(outbuf);
#define g() (((st == en) && (en = (st = inbuf) + fread(inbuf, 1, 1024, stdin))), (st==en)? EOF : *(st++))
#define p(x) (*(outptr++) = (x))

inline int read() {
    r int x(0);
    r char ch(g());
    for(; !isdigit(ch); ch = g());
    for(; isdigit(ch); ch = g()) x = (x << 1) + (x << 3) + (ch ^ 48);
    return x;
}

void print(r l x) {
    r char out[20];
    r int len(0);
    while (x > 0) {
        out[++len] = (x % 10) | 48;
        x /= 10;
    }
    for (r int i(len); i > 0; --i) p(out[i]);
}

void merge(r const int x, r const int y) {
    if (x == y) return ;
    r const int mid((x + y) >> 1);
    merge(x, mid);
    merge(mid + 1, y);
    r int i(x), j(mid + 1), k(x - 1);
    while (i <= mid && j <= y) {
        if (a[i] <= a[j]) {
            b[++k] = a[i++];
        } else {
            b[++k] = a[j++];
            ans += mid - i + 1;
        }
    }
    while (i <= mid) {
        b[++k] = a[i++];
    }
    while (j <= y) {
        b[++k] = a[j++];
    }
    for (r int i(x); i <= y; ++i) {
        a[i] = b[i];
    }
}

int main() {
    r l n(read());
    for (r int i(1); i <= n; ++i) {
        a[i] = read();
    }
    merge(1, n);
    printf("%lld", ans);
}