diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..b30a728
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*.o
+bin
diff --git a/problems/1-two-sum/Makefile b/problems/1-two-sum/Makefile
new file mode 100644
index 0000000..ab746c4
--- /dev/null
+++ b/problems/1-two-sum/Makefile
@@ -0,0 +1,14 @@
+all: lib.o main.o
+	gcc -Wall -o bin lib.o main.o
+
+lib.o: lib.c
+	gcc -c -Wall lib.c
+
+main.o: main.c
+	gcc -c -Wall main.c
+
+run:
+	./bin
+
+clean:
+	rm *.o bin
diff --git a/problems/1-two-sum/lib.c b/problems/1-two-sum/lib.c
new file mode 100644
index 0000000..465ecb5
--- /dev/null
+++ b/problems/1-two-sum/lib.c
@@ -0,0 +1,18 @@
+#include <stdlib.h>
+#include "lib.h"
+
+int* twoSum(int* nums, int numsSize, int target, int* returnSize) {
+    for (int i=0; i<numsSize-1; i++) {
+        for (int j=i+1; j<numsSize; j++) {
+            if (nums[i] + nums[j] == target) {
+                int* res = malloc(2 * sizeof(int));
+                res[0] = i;
+                res[1] = j;
+                *returnSize = 2;
+                return res;
+            }
+        }
+    }
+    *returnSize = 0;
+    return NULL;
+}
diff --git a/problems/1-two-sum/lib.h b/problems/1-two-sum/lib.h
new file mode 100644
index 0000000..30ad7f4
--- /dev/null
+++ b/problems/1-two-sum/lib.h
@@ -0,0 +1,2 @@
+#pragma once
+int* twoSum(int* nums, int numsSize, int target, int* returnSize);
diff --git a/problems/1-two-sum/main.c b/problems/1-two-sum/main.c
new file mode 100644
index 0000000..053896b
--- /dev/null
+++ b/problems/1-two-sum/main.c
@@ -0,0 +1,35 @@
+#include "stdio.h"
+#include "stdlib.h"
+#include "lib.h"
+
+int main() {
+    int returnSize = -1;
+    
+    int nums1[] = {2, 7, 11, 15};
+    int target1 = 9;
+    int* res1 = twoSum(nums1, 4, target1, &returnSize);
+    if (returnSize == 2 && res1[0] == 0 && res1[1] == 1) {
+        printf("Test 1: passed\n");
+    }
+    else {
+        printf("Test 1: failed\n");
+    }
+    if (returnSize > 0) {
+        free(res1);
+    }
+
+    int nums2[] = {3, 2, 4};
+    int target2 = 6;
+    int* res2 = twoSum(nums2, 3, target2, &returnSize);
+    if (returnSize == 2 && res2[0] == 1 && res2[1] == 2) {
+        printf("Test 2: passed\n");
+    }
+    else {
+        printf("Test 2: failed\n");
+    }
+    if (returnSize > 0) {
+        free(res2);
+    }
+    
+    return 0;
+}
diff --git a/problems/1-two-sum/readme.md b/problems/1-two-sum/readme.md
new file mode 100644
index 0000000..6042b65
--- /dev/null
+++ b/problems/1-two-sum/readme.md
@@ -0,0 +1,9 @@
+# 1. Two sum
+
+Given an array of integers nums and an integer target,
+return indices of the two numbers such that they add up to target.
+
+You may assume that each input would have exactly one solution,
+and you may not use the same element twice.
+
+You can return the answer in any order.