求助：while语句如何退出循环

本人是菜鸟，请求各位大侠的帮助，以下是我写的一个程序，
＃include<stdio.h>

main()

{
float principal,rate,charge;
int days;

printf("Enter loan principal(-1 to end):");
scanf("%f",&princiapl);
printf("Enter interest rate:");
scanf("%f",&rate);
printf("Enter term of the loan in days:");
scanf("%d",&days);

while(principal!=-1){
interest=principal*rate*days/365;
printf("The interest charge is $%.2f\n\n",interest);
printf("Enter loan principal(-1 to end):");
scanf("%f",&principal);
printf("Enter interest rate:");
scanf("%f",&rate);
printf("Enter term of the loan in days:");
scanf("%d",&days);
}

return 0;
}

请问：当输入pincipal值是-1时，怎么让程序停止呢？也就是不让下面的

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