diff --git a/sandbox/template/src/main b/sandbox/template/src/main
new file mode 100755
index 00000000..9342dd8d
Binary files /dev/null and b/sandbox/template/src/main differ
diff --git a/sandbox/template/src/main.cpp b/sandbox/template/src/main.cpp
index 0e4393ba..0a554f65 100644
--- a/sandbox/template/src/main.cpp
+++ b/sandbox/template/src/main.cpp
@@ -1,3 +1,6 @@
 #include <iostream>
 
-int main() { return 0; }
+int main() {
+  std::cout << "Hello world!";
+  return 0;
+}
diff --git a/task_01/src/main.cpp b/task_01/src/main.cpp
index 0e4393ba..7825a349 100644
--- a/task_01/src/main.cpp
+++ b/task_01/src/main.cpp
@@ -1,3 +1,22 @@
 #include <iostream>
 
-int main() { return 0; }
+#include "solution.h"
+
+int main() {
+  int sum, arr_size, t;
+  std::vector<int> input_vector;
+  std::unordered_map<int, int> indices_map;  // keeps array numbers as keys and
+                                             // indices as values behind keys
+
+  std::cin >> sum >> arr_size;
+
+  for (int i = 0; i < arr_size; i++) {
+    std::cin >> t;
+    input_vector.push_back(t);
+  }
+
+  std::pair<int, int> sol = solution(sum, input_vector);
+  std::cout << sol.first << ' ' << sol.second;
+
+  return 0;
+}
\ No newline at end of file
diff --git a/task_01/src/solution.h b/task_01/src/solution.h
new file mode 100644
index 00000000..7803b04e
--- /dev/null
+++ b/task_01/src/solution.h
@@ -0,0 +1,36 @@
+#pragma once
+#include <unordered_map>
+#include <vector>
+
+/*
+
+Output - indices of two numbers, which sum is equal to needed number, if there's
+no such numbers, the output is "-1 -1"
+
+Input:
+
+10
+10
+-2 2 3 3 5 8 11 13 14 15
+
+Output:
+
+1 5
+
+*/
+
+// Solution below has a time complexity of O(n) and memory complexity of O(1)
+
+std::pair<int, int> solution(int sum, std::vector<int> vector) {
+  if (vector.size() < 2) return {-1, -1};
+  int left_pointer = 0, right_pointer = vector.size() - 1;
+  while (vector[left_pointer] < vector[right_pointer]) {
+    if (vector[left_pointer] + vector[right_pointer] > sum)
+      right_pointer--;  // move right pointer to the left by one
+    else if (vector[left_pointer] + vector[right_pointer] < sum)
+      left_pointer++;  // move left pointer to the right by one
+    else
+      return {left_pointer, right_pointer};
+  }
+  return {-1, -1};  // in case there are no such numbers
+}
\ No newline at end of file
diff --git a/task_01/src/test.cpp b/task_01/src/test.cpp
index ef5a86ae..36bc7436 100644
--- a/task_01/src/test.cpp
+++ b/task_01/src/test.cpp
@@ -1,8 +1,30 @@
 
 #include <gtest/gtest.h>
 
-#include "topology_sort.hpp"
+#include "solution.h"
 
-TEST(TopologySort, Simple) {
-  ASSERT_EQ(1, 1);  // Stack []
-}
+TEST(solution, simple) {
+  std::vector<int> v1 = {-2, 2, 3, 3, 5, 9, 11, 13, 14, 15};
+  std::pair<int, int> p1 = {-1, -1};
+  ASSERT_EQ(p1, solution(10, v1));
+
+  std::vector<int> v2 = {-2, 2, 3, 3, 5, 8, 11, 13, 14, 15};
+  std::pair<int, int> p2 = {1, 5};
+  ASSERT_EQ(p2, solution(10, v2));
+
+  std::vector<int> v3 = {};
+  std::pair<int, int> p3 = {-1, -1};
+  ASSERT_EQ(p3, solution(0, v3));
+
+  std::vector<int> v4 = {1};
+  std::pair<int, int> p4 = {-1, -1};
+  ASSERT_EQ(p4, solution(1, v4));
+
+  std::vector<int> v5 = {1, 2};
+  std::pair<int, int> p5 = {0, 1};
+  ASSERT_EQ(p5, solution(3, v5));
+
+  std::vector<int> v6 = {1, 2, 2, 2, 4, 5, 6, 7, 8};
+  std::pair<int, int> p6 = {0, 5};
+  ASSERT_EQ(p6, solution(6, v6));
+}
\ No newline at end of file
diff --git a/task_01/src/topology_sort.cpp b/task_01/src/topology_sort.cpp
deleted file mode 100644
index e53f670c..00000000
--- a/task_01/src/topology_sort.cpp
+++ /dev/null
@@ -1 +0,0 @@
-#include "topology_sort.hpp"
diff --git a/task_01/src/topology_sort.hpp b/task_01/src/topology_sort.hpp
deleted file mode 100644
index 6f70f09b..00000000
--- a/task_01/src/topology_sort.hpp
+++ /dev/null
@@ -1 +0,0 @@
-#pragma once
diff --git a/testfile.txt b/testfile.txt
new file mode 100644
index 00000000..e69de29b