typescript: tsc is not recognized as an internal or external command, operable program or batch file

I am a beginner with type script. I am unable to compile the type script file.

As I hit Ctrl+Shift+B in the VS Code, i get the error "tsc is not recognised"

I used npm while downloading transcript.

C:\Users\sramesh>npm install -g typescript
C:\Users\sramesh\AppData\Roaming\npm\tsserver -> C:\Users\sramesh\AppData\Roaming\npm\node_modules\typescript\bin\tsserver
C:\Users\sramesh\AppData\Roaming\npm\tsc -> C:\Users\sramesh\AppData\Roaming\npm\node_modules\typescript\bin\tsc
C:\Users\sramesh\AppData\Roaming\npm
`-- typescript@2.1.5

Here is my tasks.json file

{
    // See https://go.microsoft.com/fwlink/?LinkId=733558
    // for the documentation about the tasks.json format
    "version": "0.1.0",
    "command": "C:\\Users\\sramesh\\AppData\\Roaming\\npm\\node_modules\\typescript\\bin\\tsc",
    "isShellCommand": true,
    "args": [],
    "showOutput": "silent",
    "problemMatcher": "$tsc"
}

Can anyone help?