CF 1561D1 Up the Strip (simplified version)

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main()
{
    
    
    ll n,p,t;
    cin>>n>>p;
    ll dp[200050],pre[200050];
    dp[1]=1,pre[1]=1;
    for(ll x=2;x<=n;x++)
    {
    
    
        ll flag=0;
        t=sqrt(x);
        dp[x]=(dp[x]+pre[x-1])%p;
        for(ll z=2;z<=sqrt(x);z++)dp[x]=(dp[x]+dp[x/z])%p;
        for(ll c=1;c<=sqrt(x);c++)
        {
    
    
            if(c==(ll)sqrt(x) && x/c<=sqrt(x))continue;
            ll len=(x/c-x/(c+1));
            dp[x]=(dp[x]+len*dp[c])%p;
        }
        pre[x]=(pre[x-1]+dp[x])%p;
    }
    printf("%lld",dp[n]);
    return 0;
}