Here is my code :

include<stdio.h>

int main() { int n,i,j,c=0; scanf("%d",&n); n*=2; int a[n]; for(i=0;i<n;i++) scanf("%d",&a[i]); for(i=0;i<n-1;i++) { for(j=i+1;j<n;j++) { if(a[i]==a[j]) { printf("%d %d\n",i+1,j+1); c++; break; } } } if(c==0) printf("-1");

return 0;

}

#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#endif

Try adding this to your code inside main function