1405 runtime error

[zys12345]

1405 always runtime error
#include "iostream"
#include "cstring"
using namespace std;
int main()
{
int n,i,j,sum,m,k=0;
char a[40];
while(1)
{
cin>>n;
if(n==0)
break;
cin>>a;
sum=1;
m=0;
for(i=1;i<strlen(a);i++)
{
for(j=0;j<i;j++)
{
if(a[j]==a[i])
{
sum=sum-1;
break;
}
}
if(j==i)
{
sum=sum+1;
if(sum>n)
m=m+1;
}
}
if(m==0)
cout<<"All customers tanned successfully."<<endl;
else
cout<<m<<" customer(s) walked away."<<endl;
}
return 0;
}