C :

#include<stdio.h>
int binary_search(int arr[], int n, int k)
{
   int i=-1;
   int low=0,high=n-1,mid;
   while(low<=high)
   {
     mid=(high+low)/2;
     if(arr[mid]==k)
     {
        i=mid;
        break;
     }
     else if(arr[mid]>k)
        high=mid-1;
     else
      low=mid+1;
  }
  return i;
} 
int main(){
	int n;
	scanf("%d",&n);
	int a[n];
	for(int i=0;i<n;i++){
		scanf("%d",&a[i]);
	}
	int key;
	scanf("%d",&key);
	int result = binary_search(a,n,key);
	if(result>=0){
			printf("%d",result+1);
			printf("\n"); 
	} 
	else{
		printf("-1");
	}
	return 0;
}