C++ :

#include <cstdio>
int main(){
    int n; if(scanf("%d", &n) != 1) return 0;
    long long a = 1, b = 2;
    if(n == 1){ printf("1\n"); return 0; }
    for(int i=3;i<=n;i++){ long long c = a + b; a = b; b = c; }
    printf("%lld\n", b);
    return 0;
}