UVA 12545 Bits Equalizer

#include<bits/stdc++.h>
#define REP(i,a,b) for(int i=a;i<=b;i++)
#define MS0(a) memset(a,0,sizeof(a))

using namespace std;

typedef long long ll;
const int maxn=1000100;
const int INF=1<<29;

char s[maxn],t[maxn];
int len;
int c10,c01,c1,c0;

int main()
{
    freopen("in.txt","r",stdin);
    int T;cin>>T;
    int casen=1;
    while(T--){
        scanf("%s%s",s,t);
        len=strlen(s);
        c10=c01=c1=c0=0;
        REP(i,0,len-1){
            if(s[i]==‘0‘&&t[i]==‘1‘) c01++;
            if(s[i]==‘1‘&&t[i]==‘0‘) c10++;
            if(s[i]==‘?‘&&t[i]==‘0‘) c0++;
            if(s[i]==‘?‘&&t[i]==‘1‘) c1++;
        }
        //cout<<c01<<" "<<c10<<" "<<c0<<" "<<c1<<endl;
        int ans=0;
        if(c01>=c10){
            ans+=c10;
            c01-=c10;
            ans+=c01+c1+c0;
        }
        else{
            ans+=c01;
            c10-=c01;
            if(c1>=c10){
                ans+=c10*2;
                c1-=c10;
                ans+=c1+c0;
            }
            else ans=-1;
        }
        printf("Case %d: %d\n",casen++,ans);
    }
    return 0;
}