CodeForces 703B Mishka and trip

#include<stdio.h>
#include<string>
#include<cstring>
#include<queue>
#include<algorithm>
#include<functional>
#include<vector>
#include<iomanip>
#include<math.h>
#include<iostream>
#include<sstream>
#include<stack>
#include<set>
#include<bitset>
using namespace std;
const int MAX=100005;
typedef long long ll;
int n,k,C[MAX],used[MAX],K,cnt;
int main()
{
while (scanf("%d%d",&n,&k)!=EOF)
{
cnt=1;
ll sum=0;
memset(used,0,sizeof(used));
for (int i=1; i<=n; i++)
{
scanf("%d",&C[i]);
sum+=C[i];
}
ll Ans=0;
for (int i=1; i<n; i++)
Ans+=C[i]*C[i+1];
Ans+=C[1]*C
;
for (int i=1; i<=k; i++)
{
scanf("%d",&K);
if (K==1)
{
Ans+=C[K]*(sum-C[K]-C
-C[K+1]);
if(used
) Ans+=C[K]*C
;
if(used[K+1]) Ans+=C[K]*C[K+1];
}
else if (K==n)
{
Ans+=C[K]*(sum-C[K]-C[K-1]-C[1]);
if(used[K-1]) Ans+=C[K]*C[K-1];
if(used[1]) Ans+=C[K]*C[1];

}
else
{
Ans+=C[K]*(sum-C[K]-C[K-1]-C[K+1]);
if(used[K-1]) Ans+=C[K]*C[K-1];
if(used[K+1]) Ans+=C[K]*C[K+1];
}
used[K]=true;
sum-=C[K];
}
cout<<Ans<<endl;
}
return 0;
}