coderforce 545B. Equidistant String

#include<stdio.h>
#include<string.h>
#include<algorithm>
using namespace std;
const int maxm=1e5+10;
char s1[maxm];
char s2[maxm];
char s3[maxm];
int main()
{
while(scanf("%s%s",s1,s2)!=EOF)
{
int len=strlen(s1);
int cnt=0;
int sum=0;
for(int i=0;i<len;i++)
{
if(s1[i]!=s2[i])
{
cnt++;
}
}
if(cnt%2!=0)
{
printf("impossible\n");
}
else
{
cnt/=2;
for(int i=0;i<len;i++)
{
if(s1[i]!=s2[i]&&sum<cnt)
{
s3[i]=s1[i];
sum++;
}
else
{
s3[i]=s2[i];
}
}
}
printf("%s\n",s3);
}
return 0;
}