func_code_string: #include <bits/stdc++.h>   using namespace std;   const int MOD = (int)1e9 + 7;   const int MOD2 = MOD + 2;   const int INF = (int)1e9;   const long long LINF = (long long)1e18;   const long double PI = acos((long double)-1);   const long double EPS = 1e-9;   inline long long gcd(long long a, long long b) {     long long r;     while (b) {       r = a % b;       a = b;       b = r;     }     return a;   }   inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }   inline long long fpow(long long n, long long k, int p = MOD) {     long long r = 1;     for (; k; k >>= 1) {       if (k & 1) r = r * n % p;       n = n * n % p;     }     return r;   }   template <class T>   inline int chkmin(T& a, const T& val) {     return val < a ? a = val, 1 : 0;   }   template <class T>   inline int chkmax(T& a, const T& val) {     return a < val ? a = val, 1 : 0;   }   inline long long isqrt(long long k) {     long long r = sqrt(k) + 1;     while (r * r > k) r--;     return r;   }   inline long long icbrt(long long k) {     long long r = cbrt(k) + 1;     while (r * r * r > k) r--;     return r;   }   inline void addmod(int& a, int val, int p = MOD) {     if ((a = (a + val)) >= p) a -= p;   }   inline void submod(int& a, int val, int p = MOD) {     if ((a = (a - val)) < 0) a += p;   }   inline int mult(int a, int b, int p = MOD) { return (long long)a * b % p; }   inline int inv(int a, int p = MOD) { return fpow(a, p - 2, p); }   inline int sign(long double x) { return x < -EPS ? -1 : x > +EPS; }   inline int sign(long double x, long double y) { return sign(x - y); }   int modulo(int a, int b) { return (a % b + b) % b; }   long long v1, v2, t, d;   long long dp[110][2000];   long long solve(long long i, long long cur) {     if (i == t) {       if (cur == v2) {         return dp[i][cur] = cur;       } else {         return dp[i][cur] = -LINF;       }     }     if (dp[i][cur] != -1) {       return dp[i][cur];     }     long long ans = -LINF;     for (int j = -d; j <= d; j++) {       if (cur + j > 0) {         ans = max(ans, solve(i + 1, cur + j));       }     }     return dp[i][cur] = ans + cur;   }   int main() {     ios::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     ;     cin >> v1 >> v2 >> t >> d;     memset(dp, -1, sizeof(dp));     cout << solve(1, v1) <<   n ;     return 0;   }   