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题目链接：AtCoder Beginner Contest 143 B-TAKOYAKI FESTIVAL 2019

题解

模拟即可

#include<cstdio>
#include<algorithm>
using std::max;
#define reg register
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++)
static char buf[100000],*p1=buf,*p2=buf;
inline int read(void){
    reg bool f=false;
    reg char ch=getchar();
    reg int res=0;
    while(ch<'0'||'9'<ch)f|=(ch=='-'),ch=getchar();
    while('0'<=ch&&ch<='9')res=10*res+ch-'0',ch=getchar();
    return f?-res:res;
}

const int MAXN=50+5;

int n;
int d[MAXN];

int main(void){
    reg int i,j,ans=0;
    n=read();
    for(i=1;i<=n;++i)
        d[i]=read();
    for(i=1;i<=n;++i)
        for(j=i+1;j<=n;++j)
            ans+=d[i]*d[j];
    printf("%d\n",ans);
    return 0;
}