80分求助

#include<bits/stdc++.h>
using namespace std;
int main()
{
	int n,m,i,j;
	cin>>n>>m;
	long long a[n+1][m+1],b[n+1][m+1],c[n+1][m+1];
	for(i=0;i<=n;i++)
	{
		a[i][0]=0;
		b[i][0]=0;
		c[i][0]=0;
	}
	for(i=1;i<=m;i++)
	{
		a[0][i]=0;
		b[0][i]=0;
		c[0][i]=0;
	}
	for(i=1;i<=n;i++)
		for(j=1;j<=m;j++)
			cin>>a[i][j];
	for(i=1;i<=n;i++)
	{
		b[i][1]=a[i][1]+b[i-1][1];
		c[i][1]=0x8000000000000000;
	}
	c[1][1]=a[1][1];
	for(j=2;j<=m;j++)
	{
		if(b[1][j-1]>c[1][j-1])
			b[1][j]=b[1][j-1]+a[1][j];
		else
			b[1][j]=c[1][j-1]+a[1][j];
		for(i=2;i<=n;i++)
		{
			if(b[i-1][j]>=b[i][j-1] && b[i-1][j]>=c[i][j-1])
				b[i][j]=b[i-1][j]+a[i][j];
			if(b[i][j-1]>=b[i-1][j] && b[i][j-1]>=c[i][j-1])
				b[i][j]=b[i][j-1]+a[i][j];
			if(c[i][j-1]>=b[i-1][j] && c[i][j-1]>=b[i-1][j])
				b[i][j]=c[i][j-1]+a[i][j];
		}
		if(b[n][j-1]>c[n][j-1])
			c[n][j]=b[n][j-1]+a[n][j];
		else
			c[n][j]=c[n][j-1]+a[n][j];
		for(i=n-1;i>0;i--)
		{
			if(c[i+1][j]>=b[i][j-1] && c[i+1][j]>=c[i][j-1])
				c[i][j]=c[i+1][j]+a[i][j];
			if(c[i][j-1]>=c[i+1][j] && c[i][j-1]>=b[i][j-1])
				c[i][j]=c[i][j-1]+a[i][j];
			if(b[i][j-1]>=c[i][j-1] && b[i][j-1]>=c[i+1][j])
				c[i][j]=b[i][j-1]+a[i][j];
		}
	}
	if(b[n][m]>c[n][m])
		cout<<b[n][m];
	else
		cout<<c[n][m];
	return 0;
}