#include <iostream>
using namespace std;
int main()
{
int a,b,c,sum=0,d;
a=1; b=1;
cin>>d;
sum=a+b;
for(int i=3; i<=d; i++)
{

c=a+b; sum=sum+c;

a=b; b=c;
}
cout<<sum<<endl;
return 0;
}

include <iostream>
using namespace std;
int main()
{
int a,b,c,sum,d;
a=1; b=1;
cin>>d;
sum=a+b;
for(int i=3; i<=d; i++)
{

c=a+b; sum=sum+c;

a=b; b=c;
}
cout<<sum<<endl;
return 0;
}

直接通过变量迭代实现……

#include<stdio.h>
int main()
{
    int a=0,b=1;
    int sum=0;
    int i;
    int n;
    scanf("%d",&n);
    for(i=0;i<n;i++)
    {
        sum+=b;
        b+=a;
        a=b-a;
    }
    printf("%d",sum);
    return 0;
}

#include <stdio.h>
//斐波拉契 i hate you!
int main() {
int a = 1, b = 1, c;
unsigned int n;
scanf("%ld", &n);
unsigned int sum = 2;
if (n == 1)
printf("1");
if (n == 2)
printf("2");
if (n > 2) {
for (int i = 3; i <= n; i++) {
c = a + b;
sum+=c;
a = b;
b = c;
}
printf("%ld", sum);
}

return 0;
}

#include <iostream>
using namespace std;
int main()
{
int a,b,c,xs,x=2,m=1,d;
cin>>d;

a=1;
b=1;

while(m<=d-2)
{
c=a;
a=b;
b=a+c;
m++;

xs=b;
x=x+xs;
}

cout<<x;

return 0;
}