Cards with Numbers

#include<stdio.h>
int main()
{
    int i,n,m=1,t,a[600001],b[5001]={0};
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=2*n;i++)
    {
        scanf("%d",&t);
        if(b[t]==0)b[t]=i;
        else {a[m]=b[t],a[m+1]=i;b[t]=0;m+=2;}
    }
    if((m-2*n)!=1)printf("-1");
    else 
    {
        for(i=1;i<=2*n;i+=2)
        printf("%d %d\n",a[i],a[i+1]);
    }
    return 0;
}

http://codeforces.com/contest/254/problem/A