get rid of rest input getchar &scanf

#include <stdio.h>
#include <ctype.h>
//most important here is how to get rid of unwanted input in scanf and getchar, lines 23-26, 56-57
int main(void)
{
    char ch;
    int n,m;
    int sc_res,flag,count;
    printf("1) +, 2) -, 3) *, 4) /,  Q for quit\n");

    while((ch=getchar())!='Q')
    {
        if(ch>'4'||ch<'1')
        {
            printf("wrong input, try again\n");
        }
        else
        {
            flag  = 1;     //*****
            count = 0;
            while(scanf("%d %d",&n,&m)!=2)
            {
                //scanf("%*[^\n]");//next 3 lines equals? and gets rid of bufer?threat?, so that next time scanf
                                   //can take new input
                //fflush(stdin);
                rewind(stdin);
                printf("Error Input 2 numbers again\n");

            }

            switch(ch)
            {
            case '1':
                printf("%d plus %d is %d\n",n,m,n+m);
                break;
            case '2':
                printf("%d minus %d is %d\n",n,m,n-m);
                break;
            case '3':
                printf("%d multiplied by %d is %d\n",n,m,n*m);
                break;
            case '4':
                {
                    while(!m)  //if user input 0 as divider  ***
                    {
                        printf("enter non-zero value\n");
                        scanf("%d",&m);
                    }
                    printf("%d divided by %d is %d\n",n,m,n/m);

                }
            }//end of switch
        }//end of big else
        printf("1) +, 2) -, 3) *, 4) /,  Q for quit\n");

        while(getchar()!='\n')//getting rid of the rest of input,including newline ***
            continue;
    }

    return 0;

}