E. Natasha, Sasha and the Prefix Sums

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const ll MOD = 998244853;

int n, m;
ll C[4002][4002];
ll sum;
ll realSum;
ll ans;

void init()
{
    for(int i=0; i<=4000; i++) C[i][0] = C[i][i] = 1;
    for(int i=1; i<=4000; i++)
    {
        for(int j=1; j<i; j++)
        {
            C[i][j] = (C[i-1][j-1] + C[i-1][j])%MOD;
        }
    }
}

int main()
{
    init();
    scanf("%d%d", &n, &m);
    for(int i=n; i>=1 && i>=n-m; i--)///枚举前缀和最大为i时
    {
        sum = (C[n+m][n-i] - realSum + MOD) % MOD;///前缀和最大为i的排列个数
        ans = (ans+(sum * i) % MOD)%MOD;
        realSum = (realSum + sum)%MOD;
    }
    printf("%lld\n", ans);
}