萌新求教。。WA了三个点。。哪里错了orz

#include<cstring>
#include<algorithm>
using namespace std;
const int INF=1000010;
const int MAXN=4010;
int f[MAXN][MAXN];
int a[MAXN][MAXN];
int n,m,ans=INF;
int main()
{
	//freopen("text.in","r",stdin);
	//freopen("text.out","w",stdout);
	scanf("%d %d\n",&n,&m);
	for(int j=1;j<=m;j++)
	{
		for(int i=1;i<=n;i++)
			scanf("%d ",&a[i][j]);
		scanf("\n");
	}
	memset(f,INF,sizeof(f));
	for(int i=1;i<=m;i++)
		f[1][i]=a[1][i];
	for(int i=2;i<=n;i++)
	{
		for(int j=1;j<=m;j++)
		{
			int minn;
			if(j==1)
			{
				minn=min(f[i-1][j],f[i-1][m])+a[i][j];
				f[i][j]=min(minn,f[i][j]);
				continue;
			}
			minn=min(f[i-1][j],f[i-1][j-1])+a[i][j];
			f[i][j]=min(f[i][j],minn);
		}
	}
	for(int i=1;i<=m;i++)
		ans=min(ans,f[n][i]);
	printf("%d ",ans);
	return 0;
}