神仙题！！！！
其实还是有一点没有搞懂，但是大体上差不多懂了.
code:
#include <bits/stdc++.h>
#define ll long long
#define N 3200020
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
const int mod=1e9+7;
int n,m;
ll fac[N],inv[N];
void Init()
{
fac[0]=fac[1]=inv[0]=inv[1]=1;
int i,j;
for(i=2;i<N;++i)
{
fac[i]=(ll)i*fac[i-1]%mod;
inv[i]=(mod-mod/i)*inv[mod%i]%mod;
}
for(i=1;i<N;++i)
inv[i]=inv[i]*inv[i-1]%mod;
}
ll C(int x,int y)
{
if(x<y||x<0||y<0)
return 0;
return fac[x]*inv[y]%mod*inv[x-y]%mod;
}
ll calc(int x,int y)
{
if(x<0||y<0)
return 0;
return C(x+y,y);
}
void turn_A(int &x,int &y)
{
swap(x,y);
x--,y++;
}
void turn_B(int &x,int &y)
{
swap(x,y);
x+=m+2,y-=m+2;
}
ll ans;
int main()
{
// setIO("input");
int i,j;
scanf("%d%d",&n,&m);
Init();
ans=calc(n+m+1,n);
int x=n+m+1,y=n;
while(x>=0&&y>=0)
{
turn_A(x,y),ans=(ans-calc(x,y)+mod)%mod;
turn_B(x,y),ans=(ans+calc(x,y)+mod)%mod;
}
x=n+m+1,y=n;
while(x>=0&&y>=0)
{
turn_B(x,y),ans=(ans-calc(x,y)+mod)%mod;
turn_A(x,y),ans=(ans+calc(x,y)+mod)%mod;
}
printf("%lld\n",ans);
return 0;
}
