I have some problem with Golang and include package. I have that scructure

```
src/
├── hello_world
│   ├── hello.go
│   └── math
│       └── add.go

```

**hello.go** file contains this code:

```
package main

import (
    "fmt"
    math "hello_world/math"
)

func main() {
    fmt.Println("Hello World")
    x := math.add(6, 5)
}

```

and **add.go**

```
package math

func add(x, y int) int {
    return x + y
}

```

and when I do `go run hello go` I see:

```
evgen@laptop:~/go/src/hello_world$ go run hello.go 
# command-line-arguments
./hello.go:10: cannot refer to unexported name math.add
./hello.go:10: undefined: "hello_world/math".add

```

**GOPATH**: 

```
evgen@laptop:~/go/src/hello_world$ echo $GOPATH
/home/evgen/go

```

How fix it? Thanks you!