1 #include 
     
       2 using namespace std; 3 #define _for(i,a,b) for(int i = (a);i < b;i ++) 4 const int maxn = 500009; 5 typedef long long ll; 6 int n; 7 ll a[maxn],c[maxn]; 8 ll ans = 0; 9 inline ll read()10 {11     ll ans = 0;12     char ch = getchar(), last = ' ';13     while(!isdigit(ch)) last = ch, ch = getchar();14     while(isdigit(ch)) ans = (ans << 1) + (ans << 3) + ch - '0', ch = getchar();15     if(last == '-') ans = -ans;16     return ans;17 }18 inline void write(ll x)19 {20     if(x < 0) x = -x, putchar('-');21     if(x >= 10) write(x / 10);22     putchar(x % 10 + '0');23 }24 void msort(int le,int rr)25 {26     if(le==rr)27         return ;28     int mid = (le+rr)/2;29     msort(le,mid),msort(mid+1,rr);30     int i = le,j = mid+1;31     int k = le;32     while(i <= mid && j <= rr)33     {34         if(a[i]<=a[j])35             c[k++] = a[i++];36         else37         {38             c[k++] = a[j++];39             ans += mid-i+1;40         }41     }42     memcpy(c+k,a+i,(mid-i+1)*(sizeof(ll)));43     k += mid-i+1;44     memcpy(c+k,a+j,(rr-j+1)*(sizeof(ll)));45     memcpy(a+le,c+le,(rr-le+1)*(sizeof(ll)));46 }47 int main()48 {49     n = read();50     _for(i,0,n)51     a[i] = read();52 53     msort(0,n-1);54     write(ans);55     return 0;56 }