The recurrence relation is quite simple. Here is my code for the same.
#include <bits/stdc++.h>
#define int long long
using namespace std;
int32_t main()
{
int a, b, arr[100001]={0}, ans=0, dp[100001]={0};
cin >> a;
for(int i = 0 ; i < a ; i++)
cin >> b , arr[b]++;
dp[1] = arr[1] , ans = arr[1];
for(int i = 2 ; i <= 100000 ; i++)
dp[i] = max(dp[i-1], (arr[i]*i)+dp[i-2]) , ans=max(ans,dp[i]);
cout << ans;
}