#include <cstdio>

const int MAXN=22,MAXM=(1<<MAXN)+2;

int N,M,re;
//int d[MAXN][MAXM];
int a[MAXN][MAXN];

inline int max(int a,int b)
{
	return a>b?a:b;
}

int main()
{
	freopen("skill.in","r",stdin);
	freopen("skill.out","w",stdout);
	scanf("%d%d",&N,&M);
	for(int i=0;i<N;i++)
		for(int j=0;j<N-i;j++)
		{
			scanf("%d",a[i]+j);
			re+=a[i][j];
		}
	printf("%d\n",re);
	return 0;
	/*
	for(int i=1;i<=N;i++)
	{
		int s=N-i+1;
		for(int j=0;j<(1<<s);j++)
		{
			int k=j,w=0;
			for(int tmp=j,p=s-1;tmp;tmp>>=1,p--)
				if (tmp&1)
				{
					k=k|((tmp&1)>>1);
					w+=a[i][p];
				}
			for(int p=0;p<(1<<(s+1));p++)
				if ((p|k)>=k)
					d[i][j]=max(d[i][j],d[i-1][p]+w);
			if (d[i][j]>re)
				re=d[i][j];
		}
	}
	printf("%d\n",re);
	return 0;*/
}