Why can't i input second number (num2) when i input first one as alphabet?

/program to add two integers and if input is not integer print invalid output/

#include <stdio.h>

int main()
{
int num1,num2,flag=0,sum;
char chek,chek2;

/* To check input value a integer or not*/

if(scanf("%d%c", &num1, &chek);!= 2 || chek != 'n')
  { 
      flag=flag+1;

  }  
if(scanf("%d%c", &num2, &chek2) != 2 || chek2 != 'n')
   { 
       flag=flag+1;

   }   
if(flag!=0)
    printf("invalid inputn");
else
 { 
     sum=num1+num2;
     printf("the sum is:%d",sum);
 }
 return 0;
}