C program for menu driven calculator

#include<stdio.h>
#include<conio.h>
 float add(float, float);
 float sub(float, float);
 float div(float, float);
 float pro(float, float);
 float num1, num2, res;
 char oper, ext='n';
 void main(){
   clrscr();
   while(ext!='Y' && ext!='y'){
     printf("\n*****MENU*****\nOperations to perform: +, -, / & *\nEnter Operation: ");
     oper=getche();
     printf("\nEnter two numbers:\n");
     scanf("%f%f", &num1, &num2);
     switch(oper){
       case '+': res=add(num1, num2); break;
       case '-': res=sub(num1, num2); break;
       case '/': res=div(num1, num2); break;
       case '*': res=pro(num1, num2); break;
       default: printf("invalid Operation!\n");
     }
     printf("%f %c %f = %f\n", num1, oper, num2, res);
     printf("Do you want to exit (Y/N): ");
     ext=getche();
   }
 }

 float add(float n1, float n2){
   return n1+n2;
 }

 float sub(float n1, float n2){
   return n1-n2;
 }

 float div(float n1, float n2){
   return n2>0?n1/n2:0;
 }
 float pro(float n1, float n2){
   return n1*n2;
 }