func_code_string: #include <bits/stdc++.h>   using namespace std;   int binarySearch(int arr[], int l, int r, int x) {     while (l <= r) {       int m = l + (r - l) / 2;       if (arr[m] == x) return m;       if (arr[m] < x)         l = m + 1;       else         r = m - 1;     }     return -1;   }   bool isprime(int n) {     if (n <= 1) return false;     for (int i = 2; i < n; i++)       if (n % i == 0) return false;     return true;   }   long long int factorial(int n) {     if (n == 0) return 1;     return n * factorial(n - 1);   }   int countDigit(long long n) {     int count = 0;     while (n != 0) {       n = n / 10;       ++count;     }     return count;   }   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     int v1, v2, t, d, sum = 0;     cin >> v1 >> v2 >> t >> d;     for (int i = 0; i < t; i++) sum += min(v2 + (t - 1 - i) * d, v1 + i * d);     cout << sum;     return 0;   }   