stores number of ones at odd and even position respectively
int one_odd = 0 , one_even = 0 ; for ( int i = 0 ; i < N ; i ++ ) { if ( s [ i ] == '1' ) { if ( i % 2 ) one_odd ++ ; else one_even ++ ; } } return min ( N / 2 - one_odd , N / 2 - one_even ) ; } }