1119 机器人走方格 V2

#include<iostream>
#include<cstdio>
using namespace std;
typedef long long ll;
#define Mod 1000000007 
ll extmod(ll a, ll b, ll &x, ll &y){
	ll d = a;
	if(b == 0){
		x = 1, y = 0;	
	}else{
		d = extmod(b, a % b, y, x);
		y -= a / b * x;
	}
	return d;
}
ll modInverse(ll a, ll b){
	ll x, y;
	extmod(a, b, x, y);
	return (b + x % b) % b;
}
void solve(ll a, ll b){
	ll num1 = 1, num2 = 1;
	for(int i = a - b + 1; i <= a; ++i)
		num1 = num1 * i % Mod;
	for(int i = 1; i <= b; ++i)
		num2 = num2 * i % Mod;
	printf("%lld\n", num1 * modInverse(num2, Mod) % Mod);
}
int main(){
	int n, m;
	scanf("%d%d", &m, &n);
	solve(m + n - 2, n - 1);
	return 0;
}