1856: [Scoi2010]字符串

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define mod 20100403
#define N 2001000
using namespace std;
typedef long long ll;
int n,m;
ll fac
;
void init()
{
fac[0]=1;
for(int i=1;i<=2000000;i++)
fac[i]=fac[i-1]*(ll)i%mod;
}
ll get_inv(ll x,ll y)
{
ll ret=1;
while(y)
{
if(y&1)ret=(ret*x)%mod;
x=(x*x)%mod;
y>>=1;
}
return ret;
}
ll get_c(ll n,ll m)
{
return fac
*get_inv(fac[m],mod-2)%mod*get_inv(fac[n-m],mod-2)%mod;
}
int main()
{
init();
scanf("%d%d",&n,&m);
printf("%lld\n",((get_c(n+m,m)-get_c(n+m,m-1)%mod+mod)%mod));
}