Submission #8888773

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Source Code Expand

#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#define int long long
#define mod 1000000007
#define for0(i, n) for(int i = 0; i < (n); i++)
#define for1(i, n) for(int i = 1; i <= (n);i++)
using namespace std;
int f1(int a, int p) {
	if (p == 1)return a;
	if (p % 2)return f1(a, p - 1)*a%mod;
	else {
		return f1(a*a%mod, p / 2) % mod;
	}
}
int cbn(int n, int r) {
	int rt = 1;
	for (int i = 1; i <= n; i++) { rt *= i; rt %= mod; }
	for (int i = 1; i <= r; i++) { rt *= f1(i, mod - 2); rt %= mod; }
	for (int i = 1; i <= n - r; i++) { rt *= f1(i, mod - 2); rt %= mod; }
	return rt;
}
signed main() {
	int a, b; cin >> a >> b;
	cout << cbn(a + b - 2, a - 1) << endl;
}

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