C :

#include  <stdio.h>
int main()
{	
	int i;	
	char ch;	
	char str[100];	
	scanf("%s", str);	
	scanf(" %c", &ch);	
	for (i=0; str[i]!='\0'; i++)	
	{		
		if (i%2==0)	
			str[i]=ch;
			
	}
	printf("%s\n", str);	
	return 0;
}

C++ :

#include<iostream>
#include<string>
#include<cstdio>
#include<cstring>
using namespace std;
int main()
{
	string n;
	cin>>n;int u=n.length();
	char k;cin>>k;
	for(int o=0;o<=u;o+=2)
	n[o]=k;
	cout<<n;
}