2020 camp-day6-N

乘法有分配律，怎么贪心都对，真妙啊

想了个贪心，一下过了好开心，结果只要贪心正确就必定过，哭了

#include <cstdio>
#define RE register
#define FOR(i,a,b) for(RE int i=a;i<=b;++i)
#define sc(n) scanf("%d",&n)
#define ll long long

using namespace std;

int n, a,b;
ll sum;

int main()
{
    sc(n);sc(a);
    FOR(i,2,n)sc(b),sum += 1ll*a*b,a+=b;
    printf("%lld",sum);
    return 0;
}
//不论合并顺序