Bit string

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;

const int MX = 500010;
const int MOD = 1e9+7;
const int inf = 2e9;
const ll linf = 2e18;

char S[MX]={'^'};
int sum[MX];

int main(){
	ios::sync_with_stdio(0), cin.tie(0);
	cin>>(S+1);

	for(int i=1; S[i]!=0; i++) sum[i] = sum[i-1] + S[i]-'0';

	ll cnt[2]={};
	for(int i=0; S[i]!=0; i++) cnt[sum[i]%2]++;

	ll ans = cnt[0]*(cnt[0]-1)/2 + cnt[1]*(cnt[1]-1)/2;

	cout<<ans<<'\n';

	return 0;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 1

Verdict: ACCEPTED

Test 7

Group: 1

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 2

Verdict: ACCEPTED

Test 12

Group: 2

Verdict: ACCEPTED

Test 13

Group: 2

Verdict: ACCEPTED

Test 14

Group: 2

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED

Test 16

Group: 3

Verdict: ACCEPTED

Test 17

Group: 3

Verdict: ACCEPTED

Test 18

Group: 3

Verdict: ACCEPTED

Test 19

Group: 3

Verdict: ACCEPTED

Test 20

Group: 3

Verdict: ACCEPTED

Test 21

Group: 3

Verdict: ACCEPTED